I look at a long list of factors when buying anything used but that is another thread.
Being in Brawley which is sandy and a lot of Powdered dirt i would question the motor since it has a stock air filter. If a compression test can be performed then i would consider it.
Obviously not run i a while so the at minimum the tank would need to be cleaned, new filter and the carb blown apart for a good cleaning.
There is a Kraken in the background so he is no stranger to spending $$ on RC's
His other ads have a HPI Baja 5B and a Kraton 8S with similar "Condition"
A brand new 5T clone aka 30deg North is $1300
In the end is it worth $800? Depends on what you want to do with it. Will need some TLC in my opinion. If you guess the motor is 50% worn out id say it's an OK buy but i haven't seen it or talked to the seller so there is that.