Prior to changing from VB to Xenforo, our forum had (for years) a classified ads forum for people to sell hobby-related items. It was pretty busy.
Because it was busy, we saw the interest people had in this so we switched to a well-known third party add-on for classifieds. It was slick and integrated into VB and had categories, etc. You could upload pictures into a gallery and so on. At first, I was excited about having it.
But, we found a decline in activity. Although it worked great and looked great, it was better when just operated as a simple forum. Eventually, the Classifieds add-on was almost dead (no activity) so we dumped it and went back to just a plain forum called "Swap Meet." Suddenly, it sprang back to life.
I think it would be neat if there were an add-on in which, if turned on in a particular forum, when people create a thread, there were simply extra fields for a price for an item and a PayPal ID.
I'm selling this thing, I want $20, and this is my PayPal ID.
A member wants it, clicks to pay, and the thread is automatically closed or marked as sold or something like that.
I think this would be much better than a separate add-on for classifieds as I believe that too many add-ons cause the site to become fragmented and it causes the opposite effect of what site operators want.
Just an idea...