KBB has the used 2008 Yamaha XT250 in good condition for Trade-In listed at $1900 and Retail at $2,810 as you would see on a Yamaha dealership show room floor. Back in 08, you could pick them up new for $3,400 before any negotiation with some leaving dealership floors at ~ $3K even!
$3,200 for an 08 in Good Condition w/ 455 miles on the ticker (Good is showroom for a bike, anything less and it drops) appears to be about $800 over what you should consider paying.
If I were set on the XT 250, I would make an offer at $2,400 and leave it at that. There have been used WR250Rís that have been picked up for the price of that Craigslist XT250 and the WR is a lot more bike for the money.
Regarding running lean, you should consider that a plus as it helps with lowering your fuel consumption and keeping the soot emissions low unlike those messing with the larger jets. Going rich just wastes it. In my rides, < .25 miles on the choke and it was ready to run