When Eduardo’s roommate moved out of his New York City dorm room before the summer term was finished, Eduardo saw a business opportunity. Here was an empty bed in a city where hotel rooms typically cost more than $200 per night. Renting it on Airbnb made so much sense that it didn’t even seem unusual. “I figured that renting out dorm space would just be a thing to do,” he says.

He listed the extra bed for $80 per night. The photo on his Airbnb page shows two twin beds, with a few feet of space between them. “About 150 sq. ft.,” the description reads, adding, perhaps optimistically, “with ample closet space.” So far, he estimates that he’s made about $400.

Airbnb has more than 2 million listings, including 1,400 castles. At least a handful of them are rooms listed inside of student housing. Renting out a dorm bed is unlikely to produce a windfall of cash, but with tuition costs having risen by 46% between 2001 and 2012, it’s understandable that some students are looking for extra income. According to his campus’s website, Eduardo’s tiny twin bed in his tiny square room costs him almost $9,000 per school year.

He’s rented his extra bed to people outside the expected college-student demographic. A recent graduate took the room while he was in New York to attend training for a new job. The father of an incoming freshman slept there while his son was at orientation. And a couple rented the room while they waited for a long-term lease elsewhere to begin. “To be honest, think twice before you make your final decision to stay if you are a couple who want your own private space,” the review from the couple reads. “Eduardo is very respectful, though, he will do as best as he can to make you feel comfortable.”

Campus housing buildings typically post security guards in the lobby, which makes the logistics of renting a dorm on Airbnb complicated. Sometimes Eduardo is able to get guest passes, but other times he physically needs to be with his guests every time they enter the building in order to sign them in. “I think I can bring guests in and out whenever I want,” he says. “There’s not a limit to it or a specific kind of guest they don’t accept, because that would be discrimination.”

When I called his campus’s housing department to ask whether it was indeed permissible to rent a room in a dorm via Airbnb, the man who answered the phone had a different perspective: absolutely not. “I don’t even know which reason to start with,” he responded when I asked why Airbnb-ing a dorm room was not allowed. While the campus does allow students to sublet some types of student housing, he said, “If you’re not signing paperwork to make the sublet legal, you’re an illegal tenant.”

A residence hall in Berkeley, California, where I found another dorm room for rent, had a similar reaction. The housing admissions department told me that they had just been made aware of the listing, which violates the housing contract, and had “taken action” against the student who posted it.