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GlennOption #2 (Mantle, Aaaron, Clemente, etc.) is probably the way to go, particularly so if you're looking to sell with a quick turnaround. I'm not sure I'd agree that NM 7 is without exception the lowest you can go. As a rule, yes, but hardly without exception. Mid-grade Mantle cards from the 1950s always attract a lot of attention, and I've sold several of them at roughly $100 profit. Whatever you choose to buy, if you're looking to do it for the investment potential, I'd only buy graded cards. In general you'll do best with SGC for 1887-1947, PSA for 1948-1985, BGS for 1986-present, but PSA cards do command a slight premium for the vintage issues that attract plenty of people who aren't exclusive pre-war collectors (T206, T205, 1933 Goudey, etc.).
Edited to clarify: $100 profit on each one