postage is in a separate section - so you put the price of the saddle (or a lower price if you're using the auction) and then put how much the postage will be separately. If you login and click on sell there's a long form that guides you through it
If they are well known brand and a desirable size they will sell well. However, you will never got back even most of what you paid for it new.
I buy of ebay and have sold on most saddles if they don't fit. I have always made money by choosing good pictures (absolutly essential), well described and ending at a sensible time. You do get shafted with ebay charges though, so take that into account (usually £30-£40)