That trailer is way over priced.
Not knowing what the frame rails are made of it is hard to get a GVWR for the trailer but even if it is over built a trailer is only as strong as its weakest link.
I would think the M.H. axles are that weak link.
The problem with M.H. axles is you need to change the bearings; hub; brakes; wheels; and tires to have long term use.
Lots use the M.H. stuff but you really need to replace it as the bearings and tires are not made for long term use and from what you said it does not sound like this person has done that.
To put brakes on one axle and change both axles tires wheels and hubs you are looking at about $800 to $1000
If you wanted brakes on both axles that's another $150
Then you got the cost of a new deck. 2x P.T. pine lumber with new bolts and nuts is gona be over $200. And that is a cheap deck. If you wanted a oak beck that will be more.
So now your trailer cost is over $2500 For that kind of money you could get a nice new trailer ready to go.