Here's a useful test for these sorts of situations: put yourself in their shoes and think if their actions are reasonable. If you are afraid of being scammed by this person, why aren't they also afraid of getting scammed by you?
In other words, if you really wanted something on ebay but could not afford it, would you come up with this convoluted transaction ...
Never take an installment plan over the internet
This is almost certainly a scam. Why?
They contacted you directly and want to bypass eBay - red flag.
As you've already pointed out, the buyer could back-out and demand his money back. This sounds like a new form of of the we overpaid scam, where they overpay and request money that was never theirs back.