I decided to try Gamestop's new Retro sales through their website. I bought these two over the past week:

Since I work at Gamestop, I get 15% off on top of the usual 10% for being a Power Up Pro member. Also, when games are ordered in the store there's free shipping. Duck Tales cost me $16 total, and TMNT was $40 total. I really wanted to see if the Genesis games came with cases & manuals, or if they are cartridge only. Hyperstone Heist was only the cartridge, but it is in very nice condition. The label is clean and sharp, and looks like it's had very little use. Same with Duck Tales - it looks like it's never been used.
I'll continue to order more Genesis games, as most of the prices are reasonable with my 25% discount. Maybe I'll get lucky and some will be complete. Plus, I can always return it to my store if I don't like the condition of something I receive. The shipping boxes they use are large enough to fit an original case, so I don't think that would be an issue. Plus, since these aren't sold in stores, space constraints shouldn't be an issue at the warehouse to the point where they would discard cases like some stores do for certain games.