Even though this is nearly a six year old thread I'd like to add some things from my experience as a customer on the selling end. As the mechanic for a pretty good sized landscape contractor that was downsizing it was my job to make the vehicles and equipment being sold as presentable as possible to bring top dollar for my employer. Knowing that onsite operation and inspection would be minimal there are things that can be done to hide problems with machines. In addition to bidders not doing any homework to make sure they knew the real worth of equipment they were buying.
I watched people buy our used up two axle utility trailers for $2000 when a brand new one could be bought for under $2500. It is really tough to discover non-functioning 4wd or hydrostats that are weak under load. I had a Bobcat MT-52 that had a dead cylinder and smoked really bad. So I backed off the delivery valve in the injection pump so it wouldn't inject fuel into that cylinder no more smoking. Sure it ran on two cylinders but since it wasn't ever run under load it brought as much money as the machine next to it. I had a NH TC-35 that needed transmission seals I plugged the holes in the bottom of the clutch housing so it wouldn't drip. We had a Bobcat 337 excavator that had lost the crank pulley nut and spit off the crank pulley, to fix it properly I would have replaced the crankshaft, but for the auction JB Weld and Loctite did the job. The only stipulation from the auction company was that the piece needed to start and run under its own power to make it across the block, unless it was non-powered of course. Our pick-ups got wash jobs and new factory seat covers, that $200 per truck plus my time more than paid for itself I'm sure.
The only thing the auction company would take care of after the sale that I know of is when the bidder came to pick the equipment up if the battery was dead they would install a new battery, of course as the seller that came out of our end.