Hot Springs is one of the best brands (what I have), so it is realistic to expect it to last 15-20 years.
Unless he can produce a receipt, I think him paying 10k for that one in 2007 is total BS. I'd say the spa if it is in good working order is worth more than $2800 to you, however for him to take it with him will cost him quite a bit.
A-He'll have to hire and electrician to disconnect and then reconnect the wires. Which is not cheap. Can range from $500-1000 to do this.
B-He'll have to hire someone to move it. Also, not cheap.
I'd suggest negotiating the price with him. When buying a home, there is significant wiggle room and I'm sure stuff will pop up on your inspection. It's gonna cost him alot more to take that thing with him. A quick google search showed similiar spas listing for over 4k.
Might consider calling the local hotspings dealer and see about getting it inspected. Probably wouldnt cost much but could give you a better idea on the condition. Parts can be expensive.
Having a tub set up in the yard is a blessing, especially in the fall/winter. It's really no more maintence than having goldfish, just feed it a few times a week and change the water couple times a year.
I speak from a position of knowledge here.