Is it possible to purchase a single item on eBay, and within transaction draw some of the funds from a bank account and other funds from PayPal credit?
It sounds basic, but I’ve not found out how to accomplish this, if possible.
From what I know, this cannot be done the way you are describing. However, I think you can work around it by using one source of payment to purchase ebay gift cards and redeeming them. Gift cards work more like coupon codes/discounts during the checkout process so you can combine ebay gift cards and whatever primary method of payment you wish.
If you want to purchase an item that is say $100 but you only have $50 in your PayPal it is possible for PayPal to detract the remaining $50 from your bank account if you have the accounts linked. AFAIK this is the only way you can have a somewhat split payment.