PayPal occasionally investigate suspicious transactions (putting the funds on hold and sometimes reversing them), so I wonder if it's possible to request that they investigate a payment (or payments)? Just on the off-chance a stolen credit card was used or something. Check the payment notification - does the buyer name and addressee name match? (That alone isn't dam-ing evidence, as lots of people send items to other people as gifts, but it could help with deciding how to proceed). 

I personally can't see any benefit to using two IDs whether someone was intending to commit fraud or not. At best, it seems like a "hedge your bets" type of action (i.e. if one ID is deregistered, maybe the other will last long enough to get an item or two). 

The only remotely "legitimate" (and I use that term loosely) reason I can think of is that you have some blocks in place that prevented them from buying with other IDs (for example, blocking buyers from making more than X number of purchases within 7 days). Even if that were the case, there would be no reason not to contact you and request an exemption, plus using other IDs to circumvent blocks is a breach of policy (go to My eBay > Account > Site Preferences > Buyer Requirements and look at the activity log. If you see a member attempted to purchase the sold items not long before the were sold to this buyer, you may have legitimate grounds to get eBay to cancel the transactions).

eBay's Money Back Guarantee is just eBay buyer protection re-marketed (get the item you ordered or get your money back), and for a seller to be protected you need proof of delivery, which pack and track should provide (if delivered).