I was the manufacturer of Scooter Trailers... see
http://www.scootertrailer.net.We have made over 20,000 of these trailers in the past 30 years, but, no more. I sold the last one recently and we shall make no more and I am retiring.
I am getting older and don't like working any more and I don't have to. Also after a couple of heart attacks and now two strokes, I think its time to slow down.
Those Harbor Freight rigs with car top carriers have been used from time-to-time and I guess they work from fair to okay, but, no better than that. I pulled one at a rally a couple of years ago and couldn't wait to return to the camp site and give it back. Short tongue, way too light, tiny 8" wheels and flimsy everything made it pull terrible... something you might not notice if you didn't know how well a good one can pull.
Don't go with less than 12" wheels, I put 13" aluminum wheels with steel belted radial tires on mine and you can't tell you are pulling a trailer.
Gas mileage will be reduced a little to a lot depending on the load and speed.
Some people said the tongue on my trailers looked long, but, they pulled fabulously. The bigger wheels and good tires helped also.
With a welded steel frame and fully welded aluminum body my trailers are very stiff and stout and still only weigh about 100 pounds empty.
QUALITY IS EVERYTHING! A flimsy cheaply made trailer will give you just what you paid for and no more. Some of the super expensive fiberglass trailers are WAY too heavy running over 250 pounds empty and fiberglass will always give problems over time, cracking, splitting, coming loose from the frame, etc.etc.etc., and swoopy looks just make them hard to load.
I have pulled my trailers (many of them) over 400,000 miles all over the US, Canada and Mexico without a single problem... I wish you the same good luck.