Did I make a mistake in Financing? How long do you finance?

As long as he can afford the payments and gets a lot of use and enjoyment from the trailer, then selling it would be a bad idea. Cut your losses is a bad idea IMO. You don't incur the loss until you sell it. If you can't afford the payment or don't use it then you should sell it. Everyone's situation is unique and one size does not fit all. There are way worse things that people spend money on than a trailer payment.