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That trailer looks, and sounds like a "field find" that is going to require a complete rebuild. Don't let the new paint in the interior fool you. No way of knowing without doing a compelte inspection. Go to the Portal tab and download the Trailer Inspector's checklist for guidance. There is also a utility to help you find volunteer inspectors, who might be able to help you to thoroughly evaluate the trailer.
As to the price, well, I am sure there are folks out there who will pay $3000 for it. I figure it is worth around $2000, but if it is as neglected as I presume it is, then what you pay for it is just the downpayment on the additional thousands of dollars that you will spend rebuilding everything, so it doesn't matter all that much.
If you think you are going to buy this trailer and be camping by the end of the summer, then I would say keep looking, and expect to pay around $12k.