Keith –
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The question really becomes what is your goal in selling cards. Are you trying to turn a profit, get rid of clutter, fuel your collecting habit (that's the one for me), or is there something else?
Where you sell makes a difference as well, ebay (make sure you cover all the fees or you'll more or less send free cards), here, local shows, to your friends all have different answers.
To get some concrete answers, research completed listings on ebay, but don't take it as gospel, that will get you an idea of what a buyer is willing to pay. There are times that there are unsold cards at .99, that doesn't mean the card has no value, just that someone didn’t want it at that time.
As a very general rule, base, inserts\parallels tend to bring the lowest %, RC’s and G\U next and autos the most. The more popular the card, the higher % it will bring. Even high BV autos, may have to be majorly discounted to move them, sometimes a low BV auto, can bring above BV. Certain set and player collectors can really drive up prices when they compete with each other over rarer cards. Paying postage is done best on a case by case basis. If someone only wants a card or 2, you could lose money by paying postage, but if someone wants to buy more than (arbitrary number picked) $100 asking for postage, may seem petty.
There are some cards I don’t sell, because they bring more in trade than I could get in $, there are some I know my asking price is too, high but I won’t part with the card unless I get my price, there are some I could get more for if I held on a little longer, but just want to get rid of it.
Sorry for the length of the post, with no real answers, but that is my .02, (probably asking too high for that

) Hope it helps.