Is it a 'dent' or just oil canning? Oil canning
is fairly normal and wouldn't impact the boat's life span or performance to a noticeable degree.
If it's dent caused by an impact, the plastic could be compromised and you'd have to descide if it would cause a problem down the road.
Looking at the hi-res picture it looks to me like oil canning. Like Dan said, take it for a ride; if you like great, go for it. Sounds like a good price to me.