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I want to make sure I dont let a high value rarity slip by for a couple dollars
It will happen. Don't lose sleep over it. Just go forward. It takes years of experience buying, selling, and studying. And even then you will never know everything about your items. There are few "value guides" to postcards, and most card (and FDC) prices in the market are very modest compared to the overhead costs of
ebay fees and shipping costs. The postal history aspects of the used cards/covers take more study to know the details of rates, cancels and more. It depends a lot on your time, effort, and skill to describe items and it also depends on your selling philosophy of using a site like
ebay as a net-price or an auction venue. If you are dead-set to get absolute top value for every item, then you will spend a lot of time. I would be inclined to make a bulk transaction and spend my hobby time on the next collection rather than a past one.