As someone in another thread mentioned, if you mention you're buying stuff to grow MJ and they proceed to sell to you anyway, they are now at risk for knowingly contributing to your delinquency (for lack of a better term lol). Therefore, they're unlikely to probe too deep. In fact, they don't want you to explicitly say what you're doing. "Tomatoes" is the universal code-word.
Also, I'd avoid ebay/mail order, as another poster suggested. For one thing, it's near impossible to consummate an internet deal with cash, leaving a paper trail. Also, do you want grow equipment shipped to your house?