I bought a 892 last spring - if he decides to scrap it, I'd be interested in the used thumb.

I bought an older Cat excavator years ago and it did what you are saying that one is doing. We paid a cat mechanic to come out and check pressures on the pumps. It turned out the problem was it had a pattern changer for the controls and it was partially turned/stuck.  I'd get a rough quote from Deere before you have the mechanic out - it could get pricey fast.  Maybe the seller will split the cost with you so you both find out?  Get a firm purchase price from the seller beforehand in case its just something simple!