Jim -- That's why I decided to go all-in on precancels. If you don't, most of the literature will cost you more than the stamps are worth but you will never have a clue which special one of the 1000s you disposed of you should have listed individually on
ebay! I invested in the literature and now I get to spend all my spare time working out that I haven't got any of those special ones (actually, I think I've found about 3 really rare ones in 5 years or so).