They are all pretty much in the same ballpark for those four years. If you can play it and say "I really like this!", that's a reasonable price for an all original. If it were me, I'd be popping it open to look at the circuit's originality. The transformer codes would be a key, also.
I've got to commit to a 1.5 hr drive to check it out. He's selling it privately, but it's on consignment at a respected shop, so I don't think there's any monkey business going on. He suggested I call the amp guy at the shop with any questions and he'd second the originality.
man - if it's at a shop - ask to see under the "doghouse" and to see the "guts" - there are stamps inside that will show week and year of build - also look at the tube chart - there's a two letter code on it - it should start with a "O" in green ink or a "P" . The second letter is the month. if it's possible - check pot date code also. On the board itself - you should see a lot of Blue "caps" - not a lot of "Orange" ones..... Check it out - Listen to it and try different guitars..... Everything you can get your hands on - In fact - bring your own. And a couple "drive" pedals...... Run it through the "ringer" before you commit.