I would offer them $2K. It's still a 3+ year old used tub. I bought a used Jacuzzi for $1800 when the buyer wanted $2900.
There is nothing about that manufacturer, who also makes Jacuzzi tubs, that predispose them to leaking. IMO leaks are mostly due to owner abuse, i.e. exposure to freezing conditions, chemical imbalance, etc. I would ask a lot of questions, and insist on wet testing it. Check the heater's ability to heat to 104 degrees (the maximum). Download the owner's manual and go through the topside controls and check every function. Check every jet, especially the moving jets, for function. Take off the front panel and check the inside for water, noisy pumps, etc. Others here with more experience may have more ideas.