As a general rule, Snap-On does have good resale value, but it has to be in good shape too.
Etchings or markings, as well as rust or damage, can greatly diminish the value it has.
In my experience, you won't find a ton of Snap-On at garage sales, estate sales, or auctions. It isn't rare, but due to its value, you won't find a ton of Snap On laying around.
I've been reselling for 10 years, and couldn't fill a men's shoe box with all the Snap On I've found, and I specialize mostly in tools, garage stuff, etc. (Sure, I could have bought semi loads of it, but at high prices.)
Lastly, Snap On isn't what it used to be. It used to be like finding gold. Now, after all the Saturn, Pontiac (as well as a ton of other GM and Chrysler dealerships) closed the values of used Snap On (and all tools) dropped significantly.
I still have a number of SO specialty sockets listed, which now have been on for over a year, and my prices aren't unreasonable.