I agree with Frederick that I would want to check carefully in regards to the source of the odor - it might be an important clue.
Burros have a double shell, so there could conceivably be a mold or other problem between the shells - that would be a pain to deal with, so you would want to consider that in your valuation of the trailer.
Trailers do not necessarily come with an "old trailer smell." I detest odors and have bought two older Bolers (1970's) neither of which smelled "old" at all. (That said, both of the U-hauls I have been in had a slight fiberglass-related-type smell which I suspect results from the doubled surface area of "raw" glass that is not covered in Ensolite or etc. I suppose this could be the case with a Burro
too, but I would not have described it as "old trailer smell.")
Of course it *could* just be old foam, prior pet, etc. (which you would still have to deal with, so keep it in mind)
The floor on a Burro is rather more difficult to replace than that on a Scamp
(because it runs under the inside shell), so I would look carefully at that.