Anyone who thinks they could turn that into a trailer needs to look at the law on homebuilt trailers. These are now subject to IVA (Individual Vehicle Approval) like kitcars or imports, and there's a long list of things that have to be done.
One obvious thing is that any trailer has to have the large red triangular reflectors to show it's a trailer (like the East Coast Mini example doesn't have!). But there's plenty more that needs to be done before it's taken for inspection. The Trailer IVA inspection manual
is a snip at just 127 pages long.
The East Coast Mini trailer was probably made before the law on homebuilt trailers changed - although the red triangles have been a legal requirement for just the last 20 or 30 years.....