Posted By:
BrianLet me start off by saying that my intention was never to drive the prices up of the particular cards that I collect, but ultimately by buying as many cards of my great-grandfather as I can find, it does create a reduced supply. Bob pretty much nailed it. You can get away with it for just so long before others that want the card realize they are going to have to bid higher to have a shot at aquiring one.
My first T212 was $25, the last two were over $200:

My first Zeenut cost $38; the last Zeenut I bought ended up closing over $500 on eBay I think:

So, to answer the original question, collecting a large quantity of a single card does have an effect on the market. I have no intentions of ever selling any of my cards (although I would consider a trade for a 1912 Zeenut), so increased prices only hurt me. I think the supply is starting to dry up; it has been almost 8 months since my last purchase.