If you want to purchase Bitmain mainers, go straight to their website and not some 3rd party seller:
https://shop.bitmain.com/Here's my past research about that site. Someone was also asking about that site in the past.
Googling bitmain, bitmainmasters.com doesn't appear on my side. What you probably saw is most likely an advertisement.
Doing a bit of research, I'm not sure if it is a scam, but it most likely is. Here's the whois result:
Domain:bitmainmasters.com
Registrar:Network Solutions, LLC.
Registration Date:2017-04-03
Expiration Date:2018-04-03
Updated Date:2017-04-03
Status:clientTransferProhibited
@domaindiscreet.com
Looking at the registration email, I think it looks like sort of a disposable email. Though I'm not 100% sure. It also has a video of their "mining operations" on their mainpage, which is a video from 2014. They probably just youtubed "bitcoin mining" then found that video and claimed it was theirs. Also, I couldn't even find reviews of the website, or any posts about it at all on bitcoin/cryptocurrency forums. I would stay away from it.
If you're looking at buying miners, go to the reputable sellers instead.