I agree with you that a used 4' x 8' trailer is useful around the place. I use mine a lot for toting stuff and downed tree limbs.
I'd check my local Tractor Supply. I know that last year my local TSC had a 4' x 8' trailer with an expanded metal floor for about $300. Now the price of steel has gone up since then and it didn't have the wooden side panels but I'm sure those could be added for less than $225. Also, last year I bought a used 4'x 8' landscape trailer for my S-I-L for $200. It was old but very well built, new paint and treated wooden deck, spare wheel and tire, removable metal side panels ... very servicable. What I'm trying to say is that around my area he'd be asking top dollar for a used, 4' x 8' trailer.