College students (undergrads) aren’t full adults yet. They are transitioning from living under the protection of their parents to living on their own.

Throwing away things that could be sold could be a matter of (1) frustration/exhaustion with the end of the semester, (2) no place in their car / plane luggage for extra items, (3) indifference of kids who don’t know the value of a dollar (the Ballenciaga slides are literally the most expensive version of a slide you can buy — I usually get mine for less than $10).

On the street might be a better solution, but in the US you can be fined for littering (“dumping”) if you abandon items on someone else’s property. Lots of local laws in the USA are designed to maintain high property values (while subsidizing the very expensive police departments).