What are questions I should ask when buying a used pick-up truck?

There's a great website - www.cars.com - that has every tip you can imagine for buying cars.

also, kelly blue book: kbb.com / they have resources and you can price the truck based upon your location, condition and color -- maybe a stretch, but a substantial factor in the negotiation...

size matters. bed size is usually either 6 feet long or 8 feet long. longer beds mean longer truck for parking. also there are usually 3 types of cab. standard with just front seats, or bigger cab with small jump seat in back with/or without small exterior doors, or biggest with full size back seat and large doors making it a 4 door. each makes truck longer. also engine size. most makes have at least two types or gasoline engine and one or more diesel engines. unless you are doing ALOT of hauling or towing trailers etc, stay with the gas engine, fuel easier to find, mechanics simplier and cheaper to repair. 4 wheel drive will be handy if you are in snow or rain or do off road. if off-road is an often event then an upgraded suspension may be helpful. But, if are not off roading, the suspension will beat you up on the normal roads, so depends on amount of time off road versus comfort. Two wheel drive will suffice for most hard surface driving and is cheaper since you get better mileage and is easier and cheaper to repair (less mechanicals to deal with)