I want to move away from lnbits and bring everything to nostr. Payments should be like private zaps.

All the payments you receive or store in zapit or any other lnbits are literally sitting on someone's server. Remember, sometimes when my server is down, you can't even open your wallet. That's a problem and it's a very difficult one, that's why we don't really have a solution yet.

By the way, building a good Marketplace on a single server isn't really that difficult because we already have multiple examples of them.

But building it entirely on the nostr protocol is a completely different game.

I will suggest you to keep using the Marketplace you're currently using. It will take some time to develop, I have some roadmap for this.