Not a lot of 20' get sent to the BI any more. Many for sale were 40 footers cut in half and doors added. Supply/demand makes 20 footers about the same price as the 40s. At least that was my experience 6 years ago.
A lot of the one-trippers are Chinese crap made to last only a few years (sometimes, only one trip, hence their name), you get what you pay for. A 30-year old container from somewhere else made out of weathering steel might actually last longer than a shiny-looking Chinese container:
https://en.wikipedia.org/wiki/Weathering_steel
Do your due diligence, a good container is supposed to survive because of the weathering steel's composition, not because of paint. Never buy a repainted container. The rusty-looking patina of weathering steel means that the composition is working. If you see a cheap container with exterior that looks like a freshly painted car, it's probably crap, and repainting containers interferes with the chemical composition of the weathering steel and hastens it's demise. The rusty-looking patina of weathering steel is the part that protects the steel from rust. To try and remove it, or cover it up, is a bad idea. When weathering steel is repainted, it allows water to get in between the paint and the steel, and that causes rust. Weathering steel is supposed to be exposed.
Last suggestion when buying a container: bring a ladder. The problems usually start on the roof.
ETA: Forgot to add, the wooden floors in shipping containers are saturated with some of the most toxic insecticides available. A newer container won't have the benefit of years of off-gassing that an older one has. There may be a placard on the outside of the container that decodes the poisons for you:
https://www.containerhomeplans.org/2016/...ontainers/