The biggest problem with such marketplaces is the fact that most of the previous ones that announced themselves as decentralized platforms, turned out to be centralized [to an extent]!
- Would you mind giving an in-depth explanation in regards to how you'd be tackling that issue?
I would like to hear it too.
But so far there are some contradiction characteristics in OP. For example you can't claim decentralization and have "accounts" or receive "fees". There is also some problem with claiming "legality" because that suggests a centralized authority that doesn't let anything illegal be exchanges on that platform, in a decentralized marketplace you can't prevent people from doing so.
Another thing I would like to know about is if this is server-less or will OP (or the centralized authority) run some servers where people have to visit to sign up (vendors) and buy stuff.