There are two basic types of used containers, ones owned by the steamship lines and ones owned by leasing companies.
Containers owned by the shiplines are usually kept in service for 8 - 14 years and have more denting and rust on them as they are in service longer than leased containers which are typically 4 to 8 years old.
Either way it is sometimes better for them to sell them off rather than pay a daily storage rate at the depot waiting for a commodity to go somewhere else, which may take a long time.
Used containers are graded based on condition.
Grade A: Great condition, wind and water tight, cargo worthy, minimal denting, no undesirable odours, good solid floor .
Grade B: Good condition, wind and water tight, a few more dents, minor floor imperfections, not quite as pretty as a grade A.
Grade C: Fair condition, wind and water tight, more than two patches, rusting, more dents than a grade B, the possibility of some interior odour, maybe more floor damage.
Grade D: Poor condition, slight breeze and light shower proof, available in many colors (as long as its rust), floor (if your lucky), gas mask required for entry.
Note: There is no universal grading system for used containers so one brokers grade A might be another brokers grade C.
The two most popular size containers in new and used are 20' and 40'. They are both 8' wide and come in either 8.5' tall or 9.5' tall ( high cube)
Doors both ends (we call double door units) are available in both 20' and 40' containers in new only.