I've bought several collections over the last 2 years, including tons of commons. The "Sold" filter feature on eBay is your best friend. Others have pointed out that the older cards may be commons, but that actually might be of interest to you if you're a set builder. My strategy is to see what lots of commons have sold for on eBay recently, and have a written price for what you'll pay each year. When you are looking at the cards, the height of a card, when measured against a stack = about 200 cards.
So you can quickly estimate how many commons, and multiply them by your buy price you developed ahead of time. I'll agree with the others, once you get to 1980 and later, hard to turn them for a profit. But many sellers want to sell everything, so you may have to tough it out. I've found the best way to sell them is to have a starter set, of X # different. Price examples- just sold lot of 575 different 1982T BB commons with some HOFers for $4...1980 lot of about 500 different commons for $14. It sucks and is a lot of work, but sometimes makes sense if you complete a purchase to get other things you want.
For any major cards, I usually offer about 1/3 off similar recent completed ebay auctions. Any more than that, and you can argue that you'd just buy exactly what you want on ebay, instead of buying someone's collection. For miscellaneous things, I don't put any, or minimal value on it.
Finally, after all this calculating, I normally take about another 10-15% off my first offer to allow a little negotiating...then try not to give it all back to account for time, gas, calculation errors, condition misjudgement, etc.
For me, it's a fun process, never know what you might find...but it's also why I've had to get up an extra hour early the last 2 days to ship cards from my ebay "yard sale" with tons of $1 items that I've accumulated...
