Another vote for checking the completed listings first. This helps me decide whether to try to sell it on Ebay or not. If there are a bunch of closed auctions on a stamp set but nobody bought it; I check the starting prices and then decide if the sellers were asking too much to begin with or if it's just too old of a set and not something people want right now.
Shipping is what always trips me up. I only sell about once or twice a year on Ebay. Usually SU stamp sets that are retired or I no longer use. I've tried taking everything to the post office in a box or padded envelope and getting it weighed first; then entering the dimensions and weights in the listing in shipping calculator part. That way the bidder has the option to plug in their zip code and see how much it would cost to ship to them. You as the seller can decide what shipping types to offer: such as First Class, Parcel Post or Priority Mail.
I've tried using only flat rate boxes but then I've had bidders complain that they "KNOW" it could be mailed to them for less so why am I charging so much? Am I trying to rip them off? I always manage to get it worked out with a buyer ( I only have "positive" feedback out there ) but it's a hassle and I wish I could figure out one easy way to satisfy potential buyers and not lose money.
As others suggested; always mention in your listing that you will combine items to save on shipping if one buyer wins multiple items. If you decide to go with flat rate boxes I would also mention that in your description portion of the listing even though it will also be mentioned in the Shipping charges section. Can't stress that enough in my opinion. MOST buyers are nice and go with the flow of however you decide to ship your stuff. There are just a few out there who seem to be complainers.
If you can afford to offer free shipping that would really tip the scales in your favor for buyers bidding on your auction versus bidding on someone else's for the same item. I can never afford to offer that so I try to start my bidding at about $2.00 less than I hope to end up getting for it. That leaves some room for bidding up, and the lower starting cost will attract buyers. I no longer put a "reserve" price for that reason. More often than not, my SU stuff will bid up to what I hoped to get for it ( about half of retail ) or higher. I've only "lost out" on that a few times. But still, the item sold so that's money in the craft budget I didn't have before and space freed up for new stuff when the old, no longer used stuff goes out the door.
Mary in NC