Without a doubt the value will depend on the item, condition, and to some extent location. Others are much more savvy to Snap-On than myself, but I will just point out a couple of things:
- Most items have a part number on them. If you punch in the part number on the Snap-On website it will give you a list price. At least from what I've seen, used price tend to be 40-60% of new for decent condition, more if the item is rare or desirable, less if it's poor condition or for things that aren't commonly used.
- Note that if the item has been replaced by a newer model, that will show up. Unless the old item is desired by collectors, it will likely drop a bit in value.
- To both make it easy for you and to extract reasonable value, you do have to group things into sets. But there is more than one way to do that. A set of 3/8" regular depth metric sockets quite marketable. A full set of 3/8" shallow and deep metric and SAE is expensive enough it will limit the audience. Selling sockets as singles won't be worth the time or the shipping costs.
- Using 50% of list price will give you a ballpark estimate of value, but I would echo the advice of others to look up item values individually. Also, look at sold items on eBay, not listings - don't forget that there's stuff that's overpriced that's not selling.
- Post photos here first for advice. I'd also bet that you'd get some instant messages from people willing to give you fair market value, avoid the eBay fees, and who would give the stuff a good home.