Don't forget that, even with a private sale, you will still have to pay your local sales tax which could add another $1100-$1200 in Flagstaff, if I read the tax tables correctly.
You can reduce it a bit but fudging the number on the bill of sale but if it's too low the dmv will check the blue book price on the bike and charge you that.
If I were you I would check out how much it would be to buy it brand new and negotiate from there with the private seller. Asking for MSRP on a bike with 2000 miles on it and bone stock is a bit crazy imo.