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Well, I would start out by demanding a discount to offset the horrible orange trim the current owner put on some of the interior components.
My rule of thumb is that $10-12k should buy a fully functional trailer that is ready to go camping. If this trailer really lives up to the seller's description, then $12k with some wiggle room probably isn't outrageous.
The problem with this trailer is that they didn't do a full rennovation of the trailer, they just did some fairly superficial "spruce-ups." It is hard to get a read on the overall condition of the exterior since there is only 2 exterior pictures, but one picture shows what looks like a couple dents in the front lower street-side corner, and the other shows a big, nasty dent in the front of the roof--very undesireable. If they haven't changed out the axle (they don't mention it), it probably needs to be done--the rubber rods in the torsion axles only last about 20 years. The refrigerator is also missing--that will set you back $6-700 for a new RV style fridge. They also say "wired for 110 inside and out," which makes me wonder if they ripped out the 12V
system--something else to check. When they say they "redid the plumbing," what does that mean? If they upgraded the lines to pex and installed a grey water tank, then great. If all they did was repair some copper lines that split in a hard freeze, then that doesn't add any value. That new toilet also looks like it is overhanging the edge of the platform it is mounted on...sloppy.
Do a thorough inspection. If you go to the Portal page and scroll down, you will find the Inspector's checklist. Use this list to fully evaluate the trailer. Pay particular attention to the condition of the subfloor immediately next to the walls, especially in the very rear, under the bathroom cabinet. Trailers this age are very prone to floor rot. If all your seller did was slap down new flooring over rotten subfloor, you may be in for a nasty surprise.
The more I look at this ad, the more I would conclude that this looks like a trailer flip. They might have picked up the trailer for a couple thousand, spread some paint around, and now they are looking to make a nice tidy profit for some rather minimal improvements. Tread carefully. Most trailers this age really need a complete rebuild, a little paint isn't going to make this thing safe to live in full time.