Dad made me one to play with when I was little. For the 1st one he bought a plastic toy car at Walmart to rob the wheels off of and used welding rod for axles.
After making the gooseneck flatbed, he made a dumpbed grain trailer...becuase I had to have a toy one that looked just like Grandpa's real one. For that one he bought tires from a parts vendor at a farm toy show. The bed raised up, had a moveable gate in the back and the tandem axles were held by an equilizer that moved. Now that I am older I realize how impressive those trailers are, being made with a chopsaw, drill press and a stick welder.
A grease zerk in the bed of a truck works great for a hitch ball. The hitch coupler on the flatbed is off a grease gun so it actually hooked on, but the grain trailer just has a pipe that slips over the "ball" on the truck.