I would absolutely start here, where there are no fees to be had. You will definitely find a buyer via Ebay, but Ebay will also take about 1/3 of your profits. I would personally buy your Mail Order babies in a heartbeat if I had those kind of funds available.
You might have the best luck waiting until tax refund time to advertise them, when many people have a little extra cash that may be burning a hole in their pocket.
As a backcard collector, I can tell you for sure that people do generally want those, as well as accessories.
The G1 artwork is so pretty, a lot of us like them just for that factor. Backcards aren't generally worth a lot, but it never hurts to offer them. Some may want to buy a pony with its card, some like me may prefer just the card.
I would start taking photos of what you have and post them here, and the community can help you put a value on everything.