I use this on one of my mainstream sites..works great, easy to customize..pretty much any feature you'd need.
It's been trouble free for me..but they have great support thru the forum.
Supports several payment gateways as well.
it's not free but only $50 and worth the $
http://www.bosdev.com/bosclassifieds/
they have a complete features list as well as a demo to check out plus the users manual is online if you want to take a look at that.
you might also want to check out the other scripts they have as well.
http://www.bosdev.com/