Delcampe is excellent, but you must know the European market, especially because Europeans prefer "Buy-It-Now" at a fixed price rather than bidding in auctions.
BidStart is very good, quite a number of exasperated eBayers have set up shop there, and I believe it now has over a million items listed. My "guess" would be the overall cost is about half of
ebay's.
However, if you want eyeballs on scarce or esoteric material, you can't beat
ebay. You strive to make regular repeat customers out of your one-time buyers, that's the secret, and that requires a certain depth in stock, not just a flurry of one-of-a-kind stamps and covers with minimal write-ups, and little or no prospects for repeat business.