As the prospective buyer you may not appreciate my answer but here it is anyway.
There are no price guides published for trailers that old. Condition is what determines the actual value of an older trailer. The trailer you described above is a vintage unit which has special value to those who appreciate older products. I would guess it is worth $10k, possibly more, if it is truly in mint condition and all the appliances work. The appliances on the old units were often better built than current products and may work fine.
An old friend of mine, Don Gradeless, from the days when I owned an Avion trailer, operates a website for Avion trailer owners.
LINKS TO EXECUTABLE FILES NOT ALLOWEDcpc.com/~drg/avion.html
(Oops, after I posted this I went back and clicked on the link I provided and found it no longer works. I was able to locate and open his website but when I copied the link from the top of my browser screen it produces the above and you can't get there from here. Enter Avion Travelcade Club in your google search and it should come up. His website is old and extensive and contains a large volume of Avion information. )
Go to his website and look at the classified section to get a better idea of actual value. I haven't owned an Avion for six years or so and haven't looked at his website in a couple of years so don't have any idea what he has listed now but I'd guess there are still a few listings.
You could also take the trailer or detailed inside/outside digital photos to an RV dealer owner/sales manager and ask what he considers it's worth.
I found a vintage Airstream, a 1966 22' Safari, in a farmers field with several windows broken out and full of farm dirt, rat droppings, and a rat carcass or two about eight years ago. I cleaned it up and did a mechanical restoration on it then gave it to my daughter. She and her husband had it redone with fine quality carpet, drapes, upholstery, and formica counter tops and the exterior polished. It has new everything from air conditioner to tires. It is probably worth $30k to $50k to a vintage Airstream enthusiast but is not for sale. My daughter considers it a keeper and a reminder of her childhood.