There may be situations where buying low and selling high does happen. However I don't think that's the norm unless you're selling antiques or rare coins. In order to buy low you have to find someone who just wants to get rid of the item or has no interest in researching the value and reselling the item.
BG Hanson mentioned asking having $7,500.00 into a machine, offering it for sale at $6,500.00 and possibly taking $6,000.00. There's currently an identical 5 year old model that's fairly well equipped being offered for sale on eBay with a starting price of $4,000.00.
https://www.ebay.com/itm/Grizzly-G0...933796?hash=item4b3ada52e4:g:~mQAAOSwuIRamHct
It will be interesting to see if the machine goes for more or less than the estimated value of his machine.
The current retail price of the same machine is $5125.00 plus shipping. That doesn't include the taper attachment, collet closer, or collets.
Personally I feel offering a maximum of 50% of the cost of a new machine or tool is the absolute top of the line when buying used for a couple reasons. First off regardless of condition there is no guarantee. The machine could fall to pieces on the way home and the buyer would have no recourse. When paying full price from a retailer I would expect at least a year or longer warranty. When buying from a private party it's pretty much "Buyer Beware".
Secondly almost every time I've gone to purchase a used tool or machine I've literally found multiples of the same item available within a few hundred miles. Most have come from professional shops that aren't emotionally attached to the machines. The prices reflect the fact that they are used, and are being sold at or near market value.