I've bought stamps on BidStart many times, but never sold anything there. I use BidStart for cheap sets and singles (sub-$5 or so) - basically the kind of stamps you won't find on eBay
because they're too cheap for sellers there to bother with. Most stamps over that amount can generally be found on eBay
and generally at better prices than I find on BidStart. Really, those are the only two sites I ever buy from, other than dealer websites.