I'd be afraid to use a credit line for collecting -- too much temptation (for me anyway; I'm a weak man). But for business purposes, it seems like it could be beneficial. My ebay business is fairly small and I don't typically have a lot of cash on hand. So if I should stumble upon a large collection of cards or books or records or whatever, and the price is right, I'd hate to miss the opportunity.
A couple months ago I bought a huge collection of those Easton Press leather bound books for 2 grand. In that case, I happened to have the cash available. But if I didn't, a credit line sure would have been helpful. I sold the collection for nearly 6 grand within a month so I would have been able to pay it off pretty quickly.
I guess we're talking about two different animals. Or at least I am.