This link is to a site that I buy prisms from for my windchimes. Great prices, decent service, and shipping is reasonable. They do have a few garden products and last night when I was looking for diamond polishing pads I noticed they have some small greenhouses.
Yes, you probably could keep it in a garage if no one in your family protests ;o)
The problem I see with them (besides potential problems with wind) is the size - 5 x 3 x 6 is too small to efficiently heat, especially for those of us in zone 7a or colder. And most likely the plastic isn't UV rated so don't expect a long life out of it if it's going outdoors.
I built a 10 x 12 GH out of PVC and UV-rated 6-mil plastic several years ago. It didn't look terribly sturdy, but it held up to a tornado that tore up Home Depot's GH roofing. I used 50-pound bags of playsand to "anchor" it since it sat on a pavement (a concrete turnaround stub area of our driveway.)
They are probably sturdy enough to sit on a porch, deck, or patio against the house in a protected place from strong winds. That way it could benefit from the heat of your house. I would use something like it to get a jump on the season with the EE's and small tropicals that have been dormant. A small space heater would keep the temps above freezing on cold nights if you threw a tarp over it. I reckon they would work nicely for starting seeds or cuttings, because you could control the humidity. .
We also put up a 8' x 12' x 7' PVC, 2 X 4, 6 MIL plastic lined with bubble wrap greenhouse. It is anchored by railroad ties laid out in a rectangle and we use an old storm door. But even in it all I strive for is keeping it above freezing and the plants against the plastic can get freezer burn on really cold nights. But it works and many of the brugs and citrus trees keep leaves, in fact that is usually where the citrus starts blooming. In 7b, I have to turn the heat off during the day and it gets too hot. The biggest problem I have is forgetting to close it up and turn the heater back on around 4:00 pm. I lost a few plants that way this past winter.
I have one similiar to this and it's great for starting seeds, etc..early on..I place it outside in the daytime and bring in the garage at night..it can get pretty warm in there during the day with the sun.. you have to watch and just unzip for venting..as far as your garage..do you have windows..I have a large garage that has 2 windows and the door has windows..when we built the house..we had the ceilings finished, but left the walls for hubby to keep him busy..he insulated well..I keep my plants there in the winter plus some in the house and haven't lost anything..actually they do well..not too hot nor cold...I was hoping for a large GH this yr...but too much going on..maybe next. Only problem, if you have as many plants as I do you can't park in it and have to put up with a hubby fussing about a messy garage..LIFE GOES ON!!LOL He'll get me a GH.