Have not heard of that brand. Before I got too interested, I'd want to know why the owners are selling. If they are replacing this relatively new spa find out why. Also ask for information on their electric bills, especially during the winter, if you have that issue to deal with in your area.
I'd also try and find out what a new one was going for two years ago, and compare that against the price they are asking for this used one.
Keep in mind that you will have to move this to your home, which could cost you a few hundred if you hire competent pros. Or are you capable of moving it yourself?
Then consider that you will need to spend up to a thousand bucks for site prep. You'll need a foundation, and correct electrical service. That could go a thousand easy if you hire the work out. Doing it yourself if you have the abliity will still cost you a a couple hundred.
Keep all of those expenses in mind when arriving at a price to offer. Also, get in and test the spa. Have the owner demonstrate the features and prove the thing is functional.
Brewman