Under 5K miles , barely three full seasons old and about $4k of upgrades/goodies?
That bike could reasonably command anywhere between $10,000 - 12,000 depending on how well informed the buyer is and how good a salesman the seller is.
If it's in very good or better cosmetic and working condition you've got an outright steal on your hands.
Steal it and steal it now, if that's the case - the seller is either loaded or very hard up for hard dollars!
Great deal!!!