I've old quite a few old games on eBay. I use small manila envelopes usually, as they are either a cartridge or a CD in a case, so don't need much protection. Not sure of the dimensions but it just barely fits an xbox one game case. I basically wrap it all the way up in packing tape to reinforce the package. eBay handles the shipping label creation and payment for you, and the process is fairly painless. They take 10% and Paypal takes a bit as well, but it's very easy.
Look up past "Sold" listings (it's a filter option when you search) and see what it's normally selling for, and price it around or a bit under that. Looking at what people who haven't sold yet have it priced for (normal search results) tells you basically nothing, as people put games up for crazy prices and let them sit for months at a time very regularly.