Take a test drive and get engine up to operating temp, so at least a 10-15 minute drive, make sure tranny isn't slipping, if it's a 4x4 check all ranges.
When you get back to the lot park the truck in a clean dry place.
Then look at the oil on the dip stick and check under the oil filler cap(condensation)
Pull the tranny fluid dip stick and check its color and smell for a "burnt smell".
Look in the rad overflow tank for any debris or foaming.
Look at the clean ground under the truck for any leaks, back the truck up after it has been sitting for a few minutes if that's easier.
If there are any concerns take it to a Ford dealer and they will do a Used Vehicle check on it, $100-$200, if the seller won't allow that then walk away.
Call local Ford dealer first to get price and appointment time.
Buying a used car is a gamble, heck buying a new car is a gamble with all the recalls, lol, time is money.
Check local Craigslist and Truck Trader website and find similar trucks, see what the "asking price" is, if you want, make a few calls and see if they are sold yet, if not, then "asking price" is too high.
The truck is 10 years old with 100k+ miles(as adsm08 said that's a line some buyers won't cross), so 70-80% of used car buyers wouldn't give it a second look.
You are the buyer so you are in control, remember the Golden Rule, "Whoever has the Gold makes the rules".
If the truck checks out make them a low offer, based on what you found from other sellers pricing, maybe $5,000-$5,500, you can't piss off a car dealer like you might a private seller, lol, so they will come back with a counter offer.
And even if you agree on a price you can still walk away if something doesn't feel right, Golden Rule.
With the new material(seals and bearings) used in engines and trans, 250-300k is not unreasonable to expect.
My '94 4.0 is just over 300k, I did do the head gasket, because of my own goof, but bottom end, cam/crank/rod bearings are untouched, front main seal as well, rear main is changed with new clutch.