First I would say do a lot of research. I've been buying enough Favre, Rodgers, and Nelson cards that I know pretty well what certain cards sell for. If you are looking at a specific card, look for it in the Sold section of eBay. That should give you a feel for what you would expect to pay for it. The set that it comes from will be a determining factor, and also whether it's a card that's needed for sets. For instance, in Immaculate or Five Star, the lower numbered parallels would be worth more than the base as not a lot of people collect the base set. But, Contenders the base autos for the rookies can pull quite a bit compared to the parallels if it's not a star player since there are a LOT of people doing the base sets. Then, if you're collecting current products, those will typically sell for more than the stuff that's been around for several years. I would say see about what you would expect to pay from past sales (or comparable sales), set the max you feel comfortable paying compared to that and base value, and if you get it, great, if not, move on to the next 
