How old is the lightly used trailer?
How was it used?
How much did the dual axle trailer sell for new?
Lightly used to me that the trailer should look as is was hardly used.
Axle rating alone is not enough info. You can have an axle rated at 3,500 lbs but that does not necessarily mean that the trailer will be rated for 3,500 lbs.
So what is the actual load rating for those trailer?
Besides tires wearing out faster on dual axle tires you must ask yourself do you need to haul 7,000lbs.
If your not going to carry loads over 3,500 lbs the single axle trailer will weigh less saving you fuel and tires, plates and insurance.
Then there is being able to get under the 10,001 lb so you will not have to have a DOT number and deal with DOT regulations.