I think that might be cool, but it might be against the main intent of the forum (to talk about and learn music production). If the site creators think it's okay I'm totally on board with this!

Where I live we have a high amount of people into music production. Craigslist is very active here in town and I have personally bought much of my gear used off Craigslist. My last purchase was a APC40 in new condition for $120. I have several licensed copies of software I am wanting to trade or sell. Previously, I have bought items off people out of state using craigslist and Paypal with no problem.

I recently bought my second license of Ableton Live Intro and will be selling my old copy(s) of Intro 8 and later this year when I upgrade. I also own multiple VST's and copies of Tracktor Pro that are collecting dust now that my TT's were stolen.
I have OP-1 and OP-2 expansion cards for an SP-808 sampler that are rare and decent stack of midi controllers that i'm sure someone could use.

Payment method should be decided by those conducting the transaction.
Paypal, Bitcoin are the obvious solutions.
Use discretion. If there is doubt then don't buy/sell. 

Most forums I have lurked over the years have such activity with their users.

I like the separate website idea for doing this, but...what's wrong with eBay and Craigslist?

Also, nothing's stopping you from DMing people and making buying/selling arrangements.

I think overall, adding a whole thread category for FS/FT would detract from the point of the forum, which is to learn about/share music. People can get educated about what kind of gear they should buy, and then go buy it somewhere else.

Imho no. ...simple reason. It would end up as not so populated mirror of KVR marketplace. I don't think we need that.