Collectors collect. There are very few of these things - sure, it's not say the Airstream trailer that Apollo astronauts used, but NASA stuff generally doesn't come up very often.
How it was used is also important. People can buy Deloreans when they come up at auction for the price of a new car. The Delorean used in the Back to the Future movies is the only ones worth a lot of money.
And back to the Airstream trailer - sure the market is small, and it was only really used to ferry astronauts to and from the launchpad (because the suits are bulky and the air conditioners take up a lot of room).
It was not. If you read the summary, it was a mobile command center and not used to ferry astronauts. It also only appears in pictures escorting the shuttle on the ground. That would have been after the shuttle landed (as the title says). The chance it was used to ferry astronauts is very, very small.
But it's probably one of the few artifacts of the space shuttle program left in private hands, and interest will be among transportation historians (who are broke), museums, and private collectors of transportation stuff. If the private collector is generous, they may engage in a restoration project with the historians who can document the entire thing for the public but in return, get an asset with a greatly increased value since it's likely more original.
And here is the problem: it has been stripped of anything related to the shuttle program on the inside. The outside still says NASA. The inside says abandoned RV from the 1980s. It was modified to be a mobile command center so there is no kitchen. There is no bedroom. The ad for the sale suggests buying this and bringing it to burning man. Renovating this trailer to be functional as a home would require major money and time. It would be far, far cheaper to buy another Airstream and custom paint it.