There seems to be a misconception by some posting in this thread on what a sea container would sell for. I have been selling them as a side-business for nearly ten years. When I started, a really nice used twenty foot box retailed for $1300-$1500. A comparable forty footer would run $2200-$2500. Remember when steel went through the roof? Prices DOUBLED at that time. They have come down over the last couple of years, but have pretty well stabilized at $2200-$2500 for a used twenty and around $3500 for a used forty. You can also buy what are referred to as 'one trip boxes'. These are containers that are built, loaded with freight, shipped to the U.S., unloaded and then sold. Basically a brand new box. These will run $3000 or so for a twenty and a little over $4000 for a forty.
Prices will vary a bit from one part of the country to the other, but these are ball-park figures. The biggest factor in the pricing is how far you are from the nearest shipping terminal. I don't have a large mark-up on my boxes, but I have to ship them 300 miles from Houston, and that is about $1000 a trip. I get a deal on freight by hiring some guys I know to back-haul them for me. The Houston container yards get $4.00 a loaded mile if their trucks deliver them.
If you are looking at a box, there is a large metal tag affixed to one of the doors. This similar to a VIN tag on a vehicle. It will tell you the name of the manufacturer and the date it was built. Most shipping companies retire boxes at around ten years of age, regardless of condition. If a box gets dented or twisted so it won't easily stack with others, no matter the age it's sold. Within reason this will not even be noticeable unless you are stacking them. If possible, check the roof for dents. While they are made from Corten steel, if water puddles on the roof, it will eventually cause a pin-hole. A coat of Kool-Seal will take care of this and help keep the interior a bit cooler in hot sunny weather. Check where the walls connect to the floor beams. This is a place prone to corrosion. Lastly, check the door seals. If the door seals are good then your container will be mouse-proof and virtually bug-proof. Door seals can be replaced, but finding new seals can be difficult.