Trying to solve a trust issue for a project

Hi all,

Firstly, please forgive me if I say something technically stupid. I am not trying to spam or be a troll, it's just not my area. Your advice would be fantastic.

I am planning a project, and it depends on the following concepts (facts? - need your advice):

1 - Payment is needed up front, with no guarantee of what you will get, or that I will even bother to send you anything.
2 - When you do get your stuff, I might also be able to spend it.

First issue: How does a buyer know I will send, once they've paid? (and they are paying in advance)

I am wondering if a smart contract can help me here.

When payment received, product dispatched, tracking sent. If valid tracking is not sent, payment should be refunded to payer.

Does this look like a valid/do-able smart contract? And can it be done for any coin/token? (sorry, this is probably contract 101 stuff, but I genuinely don't know). That is, if we have an agreement where I send you 5000 arse-coins for 1 BTC, can it enforce this? Let's say we're dealing in FIAT or Eth, or a third party to the trade items.

Second issue: How does the buyer receive the currency, and how can they know I don't hold the private key?

At the moment, I am thinking that the buyer should simply create an account, get a heads up on what the product is, and create the means to receive it. Then tell me. With no private stuff shared.

In the context of crypto, though, this could be difficult for newbies. I am also thinking of physical items to deliver their product, which they should be able to have confidence that I won't have access to. (Example: Casascius coins, although I know they were based on personal transparency).

I know I am being vague, but I hope I have been clear about the general problems I am working out.

Anyone got thoughts?