I imagine most marketplace notes would work with a single new Kind. Whether it's goods or services, the content tag could be a structure that carries all the needed information. Or perhaps clients should just pull the meta data from linked pages that have all of that information.
Discussion
Isn't there already a 'channels' Kind? This should be a channel.
I play some old school online games without an auction house or any in-game trade and a single Discord channel 'Buy' and another for 'Sell' is all you need.
However, one thing I'd like to see at the protocol level is a rate limit.
1 post per 6 hours or something configurable.
Regarding using a channel, that's an interesting idea. As for rate limiting, that would have to be instituted at the relay level in the same way spam is. Maybe you'd need to pay a small sat fee to post. That would certainly prevent most spammers.