As I have been in the towing, towbar and trailer trade for over 20 years (Autowbars BFD) I hope to pass on some information.
If buying used the 1st item to check is the structure of the trailer. Many may have been stood outside years in all weathers, the steel corroding all the time. I have seen quite a few trailers with snapped draw-bars / a frames that have been used for years just fine, been stored a while then OOPS!, where's my trailer.
Your prospective purchase has just been re-painted, WHY, if it was just to smarten it up then all well and good, but check carefully, is it hiding some defect/corrosion?
Please don't buy a trailer with any accident damage to it, you will NOTstraighten it out, I can tell you!
Second on your list is the tyres and wheel bearings, tyres easy to see if any deteroration, wear etc, bearings not so easy.
Jack up the wheel, try and rock from side to side and top to bottom. Any movement means a worn bearing. NOT always a reason for not buying tho' bearings can be replaced easily and it is not too an expensive job. However some of the old imperial sizes are becoming harder to source, and when you can get them they can be / are expensive.
Suspension, leaf springs, well they last and last, just check for broken leaves, independant torsion rubber type can fail due to the rubber inserts degrading, if the wheel stub axle is not in line with the main axle WALK AWAY, it will be expensive.
On to the coupling, if you hear any unusual knocking noises whilst towing check the coupling. They do not last forever, many get used without grease. That causes wear. Over 1mm of wear and you should replace it. Cost? From £12 for the smallest trailer up to £225 for a 3.5 tonne braked coupling.
The lighting system can be replaced in 2 ways, an inexpensive lighting board (approx £25 max) or a full rewire. The latter can be expensive, say from £75.
The UK law states that all trailers need a specific set of lights, reflectors etc, dont get caught out, they are not expensive.
All trailers require mudguards if the wheels are outboard of the main body.
The legal weight limit for an unbraked trailer is 750kgs, or 3/4 of a ton. I have seen many sellers offering trailers with massively overated gross weights when the trailer has no brakes. Maybe they just don't know the law ?????? Up to you to decide that one!
Car recovery dollys/dollies that are unbraked are mostly illegal to use on the UK roads. Add together the weight of the car being towed, and the dolly and it will come to over 750kgs
Never fit a second hand towbar to any vehicle, you dont know its history.
It may have been in an accident you dont know about. Visually it may look fine, however there may be internal cracks etc. We, Autowbars, NEVER fit second hand towbars for this reason.
Tow carefully and safely. Thanks for taking the time to read this.