This is my thoughts on what ive seen on ebay.
Buy it now or bid. Well we have all seen a item buy it now price and a starting bid and we put in a bid which deletes the buy it now price. Then the item sells more than the buy it now price. So if you really want it buy it if you can afford it at good price. Doesn't happen in all cases and sometimes you get it cheaper by bidding. Ebay is about being in the right place at the right time. You could go looking for some thing you want and maybe days before you were looking the item sold cheaper then you bought it and days later it sold dearer. So its how bad you want it or how long you can wait for a good deal.
I think selling is 100 % the same way.
You win some you lose some but it all should work out 50/50 in the end.