The problem is, Redbook prices don't always reflect what the market will do and eBay does. Sure, there are some coins that go cheap only because they were incorrectly attributed, put in the wrong category, etc. but most of them accurately reflect what the market is willing to pay. Sure, sometimes it might work but I don't think it would be a reliable plan. Indeed most coins I see on eBay go -much- higher than what I can get them at my LCS, although your results may be different.
When buying coins for potential profit (rather than simply loving the coin) you need to know:
1) How quick of a turnaround time can you get your capital back. If you're spending $300 on a coin to sell for $400 but it takes you 2-3 months, you might be better off to buy several low-valued coins with quicker turn around times that give you less profit, but you can do them more often.
2) If a coin is priced below Redbook it might have a problem that you're not seeing. What problems are associated with this "cheap" coin (weak strike, unattractive toning, artificial toning, PVC damage, cleaning, polishing, misattributed, bent, holed, etc.) and how will this affect your selling.
For me, the best place to buy coins cheap and resell them are auctions, not coin auctions but just normal estate auctions. Sure, you've got some auctions where everything will go 2-3x redbook, but then you've got others that the auctioneer has no clue what anything is worth. Stuff like silver Kennedy half dollars being sold like common clad, foreign coins going dirt cheap, rare varieties that go unrecognized, good coins buried beneath junk in lots, etc. Between that and trolling the junk/melt piles at my LCS, I can eBay enough coins to let me expand my collection.
..............you have to know what it is you're buying and that won't come easy. You will certainly lose some money before you gain anything, its the price of admission, good luck.....
