Paul, wonder in to your friendly dealership, (in a farm/agricultural/timber area) and simply ask them what kind of a hit does a pickup take value wise, IF the interior of the bed is scratched up from use. Of course, put the provisio that the exterior paint/body is in nice shape, and the interior of the cab is in nice condition.
I've had them flat tell me that it makes NO differance in the value of the rig, providing the mechanics and body, etc is in good condition.
The only reason to add the spray liner when selling (if personally selling it) is the same as the reason you spent a couple days detailing it in the first place....to get the highest price possible out of it.
Note: I've never bothered doing anything to the inside of a pickup bed...other than cleaning. Couldn't see spending 200 to make .... maybe a 100 more.
Have you seen commercially done spray liners peeling? I have, ugly too! I've also seen them leak moisture under them, and rust out an area.
I've also spent around a thousand redoing a bed with teak boards just for the 'look', but then, I wouldn't put anything back there either! One p/u I had, never, ever, hauled anything, except a custom made topper. I borrowed my Dad's truck to take stuff to the dump. Truck had less than 7000 miles on it when I sold it ... only six years old at the time. Absolutely worthless as a truck, but nice at the drags, and car shows!
The current operational theory that I operate under is: Buy it, USE it, sell it. No extra horse pucky along the way, except to add visually appealing touches, which are very minor in cost.