If you're going to sell stamps individually or in sets on
ebay (U.S.) you'll need to list the Scott Catalog number(s) for the stamps, otherwise collectors looking for them probably won't find them. Unfortunately you won't find a complete version of Scott anywhere online, even from Scott themselves. The information contained in Scott is copyrighted and trademarked and any website using information from Scott needs to be licensed by them to do so. Small bits and pieces exist on websites here and there, but a comprehensive online version of Scott simply does not exist. You'll probably need to make a trip to a library and check out the hard copies, if that's an option for you.
Another option would be to look on
ebay itself to see if you can find the same stamps that another seller ID'd (hopefully correctly). Even this can be dangerous, though, because classic-era stamps have many "look-alikes" with extremely valuable stamps being nearly identical in appearance to worthless ones. You could make the error of seeing the valuable one and assuming your stamp is that variety. This is much less of an issue for most countries after about 1950 or so, but it still happens occasionally. If yours are all pre WWII, you could run into this fairly frequently.
You could also try searching for dealers that sell the kinds of stamps you have. Some (though not many) have e-commerce websites that have pictures of the stamps along with the catalog number and price. The same caveat about look-like stamps would still apply, though.