If you're going to buy them at a store, use *cash*. And this isn't necessarily just because you could grow weed with them.
Say that your name and CC# end up on a list that the store has to give the police. They check your electricity and see a big spike, so they drop in unexpectedly and don't find weed but a basement full of reptiles that violate city by-laws.
Well, they've already spent the money on a raid, may as well come away with something.
Sounds paranoid, I know, but for every decent cop out there is at least another one who's a jerk.