eBay.
Unless there's something super-special about your Game Boy (remember, they sold MILLIONS of these), one (or more) of your titles are rare, or stuff is still sealed or mint in box, you're probably not going to be buying a new car off this. Without knowing the games or condition of anything, I'd say you'd probably get $10 for the system and $5 per game (on average). Again, if the games are more popular or rare/uncommon titles, or stuff comes with boxes/manuals/etc., you might be able to bump that up. If the system is in rough shape, the games are super-common junk, labels are torn, etc., plan to deduct from that $70 estimate.