[thecatspajamas]

You're right that you won't have any Buyer Protection through eBay if you pay that way (because as far as eBay is concerned, you didn't buy anything through eBay). Depending on how the seller invoiced you, you might still have a recourse through Paypal to reverse the charges if the deal doesn't come through. It's not as straightforward a process though, and if they just sent a "payment request" rather than an "invoice" through Paypal, you still might not have any recourse.

Basically, if it makes you uncomfortable dealing with anyone outside of eBay, don't do it. That's not to say that there is anything inherently wrong with doing business off-eBay, but if you're used to letting eBay's Buyer Protection promises override your Spidey Sense when doing deals on eBay, you need to start paying attention to it again when you step outside of that protective bubble.

Edited to add: There's nothing wrong with playing dumb or scared in a situation like this either and telling the seller you can't get the payment to work through eBay. If it's a legit sale and he wants to get paid, he'll figure out a way to either fix his eBay profile information or re-list the card with a BIN just for you. Ebay won't punish you for not following through with a purchase that the seller tries to take off-Bay.