So, I think it's important to make people aware of potential issues they will be faced with as users of the system you're proposing. Essentially I think you're proposing a system very similar to localbitcoins, but for altcoins and maybe with better reputation management (full disclosure, I've never used localbitcoins).

The big danger is that individual users, by repeatedly offering to sell or by crypto for money from the US or to US residents, will be considered to be money transmitters under fincen regulations. Probably the highest risk will be to US residents, since they are the most likely subject to an attempted prosecution.

There's already cases of this type, here's some related links I found on a quick search of the topic:

https://www.coindesk.com/localbitcoins-trader-pleads-guilty-money-transmitter-charge/
https://localbitcoins.com/forums/#!/general-discussion#money-transmitter-licenses

Please understand, I'm not a fan of any of the above, and I don't like to be the bearer of bad news, but I think it's important that people understand the risks for a legal entanglement.