I have already sold several physical items via nostr and the lightning network. I don't have direct answers for you in regards to SN specifically but maybe I can provide some insight.

My transactions have been based on trust. As far as I know, there is no concrete way to verify that I actually have knit Bitcoin beanies and that I will actually ship it to them. They trust that I do. Perhaps they see that I am an active, actual person on nostr and make their judgement from there. "Don't trust, verify" comes in when it comes to the actual exchange of Bitcoin via lightning. Because of payment hashes, we can both verify that they paid me. It is in my best interest to be honest and trustworthy if I want to continue selling in this space.

I think the Stackers Guide to Sell has a bunch of great best practices. As for getting paid lightning fast... well, lightning is pretty much as fast as it gets. The question you're really asking is "how do I sell things fast" which is a very complex answer depending on your product, your understanding of your buying audience and so on.

I've only been on SN for less than a day my opinion on your third question might be a little premature. My initial answer is that SN doesn't need to provide private nostr DM as a feature. There are more than enough other nostr clients that can do this. I personally carry on my private messaging through my preferred nostr social client and intend to keep it that way. I imagine that SN users who want to buy my beanie hats would also have their preferred way as well.