Sorry to hear of your hassle today. I am glad that you are alright now though and your investment intact.
Hopefully the seller will be reasonable, in a way at least the trailer and boat are secure on the aa truck. To be fair to you with the distance fixing the problem on route would have been difficult for you in unfamiliar surroundings.
It looks like a bramber trailer like the humber, you should be able to get hub assemblies from trailer companies, tow sure have them. And ones that match the wheel size and Pcd.
In my experience so many people take care of their boats and then leave the trailer. Those bearings have not seen any maintenance for a very long time.
Let me know how you get on. If I am close by travelling could pop in and have a look at the setup if that helps you.