I've been buying on ebay since 1999, 99.8% positive results. I've occasionally done the "best offer" thing when it was offered. Sometimes the seller already has a minimum price and if you meet or beat it, you get an instant answer and if you don't, you get an instant rejection. The other way is the offer is open and you don't get a response until the seller actually sees it.
IIRC I bought a couple of PSC brass tank cars pretty cheap that way, one damaged that was really dirt cheap. Had a couple dents in it, but the ends were not soldered on so I was able to get inside and rub out the dents. Just enough remaining to be realistic once I get around to repainting it.
The only wood kit I bought on ebay was a Quality Craft auto rack, which is borderline OT for this list. It does represent an early 85' rack. My plan was always to build the car in plastic and just use the plans and castings. Whenever I get around to that project.
I've sold some Sunshine kits on ebay, the same way I always do - $5 minimum bid and no reserve. They did well, including a couple of Naperville giveaway minikits... well not exactly giveaway considering the price of admission but, I accumulated some I couldn't use and got surprising good money for them.
I've kept a few Sunshine kits that are borderline "boomer" era cars, or just a few too interesting to part with.
Ebay is about the best place to find about anything obscure. Think of the most obscure kit, part, manufacturer... and you'll probably get a hit.