Hm...I've been thinking about getting some more semantics into the marketplace.

Benefits would be for example the ability to simply check how much things have sold for in the past or how often something has been sold.

And one click way to find all threads in marketplace by user x that are not marked [done].

I do think care should be taken w.r.t. turning this into a new economic hub ala wallstreet; if you visualise f.i. the highest selling price per board, a race to the top becomes likely, and if you do the reverse the reverse becomes likely.
Tricky thing, I have no answer to that; thoughts?

Nice, but that would be hard since it's phpbb, just forum software and no semantics except what you hack into it.

Well my experience with forum software is pretty much nill, but, a simple search query should be able to do this, at least the part I suggested:
TS/author: webwit
category: marketplace
exclude containing: [done]

Like in this thread: deskthority-related-f17/reorganising-marketplace-t2396.html ?

Hm..I have read that thread and then somehow forgot about it. m(