Surely a garage sale would be from someone's garage - the point about boot sales is they take place on a car park or somewhere like that, with loads of different people. And most of the time they use tables rather than doing it direct from the boot.

I've seen a few of what I'd call a boot sale in feature films set in America, so they must happen.

An indoor boot sale here would be called a table top sale. Tobe distinguished from a jumble sale, because that's where everything is being sold for some charitable purpose, rather than individuals or families selling on their own behalf. And then there are bazaars which are different again.