Let's look at this two ways.
1) it's a 11 year old truck, the amount of rust is almost non existant. Your paying almost 1/3 of what it costs new, your buying it from the original owner who decided to keep the truck 11 years and has good service records. The fender is a bolt on item, a non structural part of the truck, if it was a 1/4 panel I'd be a little more curious about the repair and why it was done.
2) let's say for a moment you don't want the truck and pass on the deal. You could spend 18-20k and buy say an 05-07 with maybe 90-110k miles. This imaginary truck could have 3 owners, some basic service history. Timing belt done? I'm not sure. Brakes look 50%, any other maintenance? Oh yes I've always changed the oil ever 5k miles, just put In a new battery and tires are only 18 months old. Could have more than a few scratches and maybe the whole side of the truck was painted you didn't notice because the work came out nicely and they taped over the Vin tags.
All I'm getting at is this seems like an excellent truck, it's no steal of a deal but your not over paying. One owner clean fax and the owner clearly cared a lot about the truck.