A wise man (Hi Scott Levy
) once said to me, "Try to buy the best player, from the best set, in the best condition."
Not saying I've successfully done that always, but the point is, if you're collecting T206, try for Cobb, Matty, Cy, with tougher backs...(or Wagner, Hi Ryan!
)...if you're collecting Goudey, try for Ruth, Gehrig, in the best condition you can, if you're collecting Cracker Jacks, Cobb, Matty, Jackson...if you're collecting post-war/Modern- Mantle, Jordan, Gretzky, Jeter, etc. And try to get their rookies, or most popular sets their in.
(Now add to that, Trout, Mahomes, LeBron, etc).
Seek out the best cards, of the players, in the best condition, you can get. It sounds simple/easy, but one can easily go down a collecting road that's too niche or esoteric (nothing wrong with that, but that does limit buyers if & when you decide to resell).
(PS- Just looked on eBay, seems like Jeter's SP is the next to skyrocket.)