You won't have rust on the water tanks. they are plastic. If the tires are 7 year old, they will need to be replaced, regardless of condition. Does the top go up and down easily? Do all the appliances work on both AC power/DC power/propane (depending on appliance)?
When the top goes up and down, do you see any evidence of hydraulic fluid leaks under the trailer? Hydraulic systems sometimes degrade when not used.
Do all the lights work inside and when hooked up to a tow vehicle, do all the outside lights work? Rats or mice can chew up wiring when a trailer is in storage.
There should not be any evidence of water leaks if it's been inside, but you might have to reseal the windows in a trailer that old.
I would think the battery will have to be replaced too, if it's been in the trailer for 7 years.
There's a checklist for buying trailers on the forum, but because it's been in a garage, many of the items in the list probably don't apply.
I'll add that due to Covid, the cost of used trailers, like used cars and homes has gone up some. Realize that even if the trailer is in great shape, you will find there are things you will have to repair/attend to during ownership. A trailer becomes somewhat of a "hobby". Are you "handy"? I think it's best if YOU can repair problems, rather than depending on RV repair places - they don't do good jobs in many cases. That said, most everything in a HiLo is rather generic and you can find replacements for broken things online or locally in hardware stores.
Make sure the owners show you how to operate EVERYTHIING!
- Jack