Garage Storage?

I recently had several tube and solid state amplifiers as well as receivers passed to me to sell. They've been stored in their boxes in a garage with no cooling for several years in SoCal where it gets pretty hot during the summer. I currently don't have the equipment to test out each of the systems. Assuming they were in working condition before they were stored, is there a big chance that they are damaged from the heat?
IMHO, humidity, not heat would be more of a factor in corroding the electronics, however, heat could dry out any rubber parts. Good luck.
I'd want to be able to assure buyers that the pieces are in working order despite the years of storage. Can you get some cheap thrift store speakers to test them with? An mp3 player would work as a source with a 1/8" jack to phono adapter cable.