That is a whole can of worms there Jim.
First, you have to find one that hasn't been picked over. Then you have to drop the pan to see how much crud is in the bottom. You also have to replace the pan because the yards punch holes in them to drain.
After all that, the U-Pull won't give a cash refund. They give you a voucher that is good only at that location, which puts you back to square one. LKQ still does a cash refund, but the pricing is generally higher.
No matter which way you go, buying used is a crap-shoot.