I believe US car boot sales are called garage or yard sales. It could
be that they are held by individuals at their own home. It certainly
looks like that if you google garage-sale OR yard-sale and restrict it
to the USA with the advanced search. eg you get
http://www.garagesalesource.com/
I haven't got any suitable links on www.carbootsrus.com as I haven't
found anything like our car boots. If you do find anything can you
please let me know and I'll add it/them.