Watch out for the S17 series
I only have 3 S17 Pros, purchased new direct from Bitmain. The went strong for a year and then started having board after board die. They have 3 hashboards each. 7 of the 9 boards have failed and one power supply. In sending out for repair (not easy to find or quick), 2 were determined unrepairable.
If you look on ebay, you will find quite a few non working or partially working units.
My opinion, stay away from S17 series.

Makes me worry about the S19, but just bought one a month ago.

Whatsminers seem to have a pretty solid rep.
I have one M20 I purchased from Whatsminer.net (not .com), that's been a trouble free workhorse for over a couple years now.

A little feedback on Unistar and my limited experience so far
- I am not familiar with Peace but have dealt with Unistar on Amazon

- Good rating on Amazon
- I have successfully purchased a unit form Unistar on Amazon.
     I was more set back by Amazon shipping me a 35 pound Commercial Tarp instead of a Miner at first and Amazon resolved TWO WEEKS later.
     This was an Amazon Error. Not Unistar. I did get the new unit as expected, delayed by Amazon mess up.

Though prices was a bit higher at the time than other suppliers, it was USA stock, 2 day delivery (except for Amazon screw up) and I was able to put on interest free credit Card. There is a significantly larger price gap now (much higher on Amazon than direct from most suppliers). There was a significantly smaller gap at the time I bought. The Amazon price seems to jump with every batch they sell.  Currently the Amazon prices from Unistar are way over priced.

Unistar has been communicating with me by email through Amazon and directly after I inquired directly with them about an additional purchases. They offer lower prices of course directly and in line with other supplier quotes. Amazon prices are significantly higher.

To be clear I have not bought form directly Yet but am considering.

Both list items as a "Trade Assurance" Supplier on Alibaba.
I would ask to purchase that way. I believe they can invoice you through Alibaba  (I believe there are additional fee's you pay through Alibaba).
I am exploring Alibaba as well and have the same trust issues.
They may just be trying to offer you lowest price with direct BTC payment
If they flat out deny the ability to use trade assurance through Alibaba, I would then forward a copy of communication to alibaba as they should not be listed as a Trade Assurance Seller.

Chain[1]: find 0 asic, times 2
Chain[1]: find 0 asic, times 2
Chain[2]: find 0 asic, times 2
All three boards are failing to boot and likely have bad asic chips. Way to common with S17 models.

I have one board that if I reboot maybe 6 - 8 times in a row, it comes to life. And hashes without hiccup for weeks until I have to reboot.
**You could try that just to see if you can get a board to go, but may not work and the board will still need repair eventually.

I have had 6 of 9 boards fail in three machines. I was able to get 2 repaired, 1 unrepairable, and others pending repair hopefully.
If you feel like it, you could carefully remove the boards, and may find loose or shifted heat sinks as well. That's a sign it got too hot. Loose heatsinks (completely detached), or even shifted, can sometimes be reset with a heat gun if you know what you are doing, and board MAY come back. This has worked for me a few times. But there is likely still an issue with that chip overheating in general. Shifted heatsinks, again because that chip got too hot, and heatsink did re-fuse, may not have re-fused completely or created solder balls under on chip shorting out asic.

Finding a repair facility is hard. At least has been for me as they are backlogged. I used HMtech most recently but they have stopped accepting new customers with less than 25 units in need of repair due to backlog.

I am debating ordering a unit from Mining Expert through Grace Zhou as a couple member's have vouched for Grace.
I hoping maybe another couple members can also confirm.
I really worry about Crypto payments direct to seller without Alibaba or other assurances.

Any other reviews or opinions, or safe payment options?

I have used HMTech successfully a couple months ago.
In short I would recommend as trustworthy in the US

They warned of a large backlog then and they require you to send the whole unit in to "be in line", and it took well over a month then as they warned me before.
They repaired 2 of 3 boards on an S17 Pro. They were unable to repair one of the boards.
Overall, they did what they promised at rates listed.

Unit did have one of their replaced chip's heatsinks come loose as I was re-assigning boards into other S17 Units. I was able to re-secure (with my limited skill).The repaired boards have been running fine since. I suspect they would have addressed had I contacted but I didn't want to gave to send it in again.
I would use again, unless I can find a faster service or a service that notifies you to send in unit say the week before it's "turn"

However, I can confirm they told me just a few days ago when inquiring about another repair, they are Not accepting new customers at this time with less than 25 units due to backlog. They will accept single units from past customers.

So as I wait for bitcoin to climb just a bit before finally breaking down and ordering an S17e, Bitmain anounces a cryptic Facebook post
"Antminer is unveiling something big at this year’s #WDMS2019! A groundbreaking new product that will change the industry."
Now I am on hold again
Probably a bunch of hype, and a long delay in whatever they may announce in a new product I suspect

They had M20 45 Th/s15 in stock a couple weeks ago and were down to 2 when I snagged one.
They seem to have a few units here and there that pop up for quick shipping (at a premium price of course, I paid just over $3000 with shipping).
At the moment they have 12 M20S 68 Th/s in stock 1-2 day shipping, (and of course at a premium price) These ones are listed as Demo Units.
https://whatsminer.net/product/m20s-demo2/

I have seen the same happen with Pangollinminer as well for limited stock

Based on feedback here that Whatsaminer,net is legit, I took a chance and ordered from Whatsminer.net last week for shipping from China to US.  They had a few M20 45Th/s (shipping within 5 days). I was impressed that it arrived at my doorstep (DHL) in about 7 days (Midwest US). I would note that there were no email updates or notifications from the company directly, but I received a DHL shipping notice at about day 3.

I have noticed there prices tend to be a bit higher than Pangolinminer (for same shipping times), but at the time everything on Pangolinminer was Dec delivery only.

Unit is up and running good so far.

As for the M20 itself;
The thing is a beast for weight (twice that of any of my Antminers) and louder than any Antminer I have had so far (S9's T15, S15).
It does have a low power option as well, but does NOT function. Hopefully they will update firmware sometime soon.
It is hashing at speed claimed but just a little higher power draw than advertised (but withing specs) at 49.5 W per Th/s.

T15 issues. When switching pools I keep getting Socket Connect Failed. It takes several re-boots (quite a few sometimes) and finally connects. Annoying. Now, after less than a week, I lost a chain. So now hash is under 15TH.

So testing on 1 machine for about 6 days now. Good start . On day 2, overclocked. Some Trial and error, got a steady 14.2  at 80 - 82 degrees (13.5 machine. usually ran about 13.6) .
Ran great and steady at 14.2 or so for several days, until last night. Then dropped to 13.4 -13.5. Machine re-starts daily. Tried resetting stock speeds and now machine is running at 12.9 or so. Will continue to play with....

Yes, you can just change the MAC address in uEnv.txt with notepad

Try using Rufus 3.0 instead of WinDiskImager32 to flash SD card in Windows

See Post #24 & #26 to flash an SD card with Windows