OP didn't specify what FFP was he/she interested in, but it seems of the Legacy airlines THE ONLY ONE to offer any chance for miles from eBay purchases is UA via MyPoints @ 2-4 MyPoints/US$ (depending on the month of purchase) to accumulate for 2500 Millage+ Miles (5100 MyPoints) or 5000 Millage+ Miles (10000 MyPoints) at a time w/o $ or time limits (MyPoints doesn't expire as well).

The reason UA is offering Miles is that they used to own MyPoints until they sold 'em to United Online (no relation to UA) several years ago....

If you go this way you can answer the Surveys provided (10 MyPoints if you don't qualify, 50-200 MyPoints if you do) and use the grocery savings coupons provided (by coupons.com and Smart Source, 10 MyPoints/coupons.com coupon redemption, 25 Bonus MyPoints for the 1st 10 coupons you print/month) to reduce the time needed to get enough MyPoints for an award.

So far it seems neither AA nor DL have ways to use eBay purchases to get Miles into their FFP's ...