I personally bought an expired adult domain for 15$, with
DA 16
PA 32
PageRank: 3/10
In my opinion, a domain for 500$ should offer way, way more. I guess about 40/40+ which give you by adult websites almost GOD status already:p. You can search for expired domains here; expireddomains.net Register there for free…it will show you expired domains with backlinks…if you take your time…you will for sure find some adult domain very cheap in a very good condition.
It is really hard to sell adult domains for good money these days for many reasons. Why not build a site on it and see if you can rank it yourself? Definitely worth trying out in my opinion.