Jay,
One factor you may have discounted is that of your time.
Bidding buying on Ebay, the entire recording process from prep to post, and then selling on Ebay etc whew!
I do not know the value you place on your time, how old you are, or how long you expect to live, but I think you are coming up short on several counts.
If you already owned these tapes and had paid substantially lower prices that would be one thing, but I am afraid it sounds as though you are losing money on each transaction but making up for it in volume.
If you enjoy recording and all that goes with it knock yourself out, but this sounds as though you are chasing your tail.