Thanks for the link, I tried a search but mostly was finding posts for the classifieds.
After reading the linked thread I started to wonder what criteria does one use to determine if a tools condition is excellent, good, etc? Seems like it might be pretty subjective. So far it looks like new/unused goes for 65-75% new price, great condition 55-65%, good condition around 50-60% and anything with personal markings around 25% new price. Does this sound about right for general stuff like ratchets, breaker bars, sockets, etc. and not vintage or rare collectable type items?
Enrare:
I think you started something here. Coins and comic books are rated on a precise numeric scale (according to Pawn Stars), so why not tools?
Keep in mind that retail prices are often discounted 40 percent or more. Later, if a tool ends up on the bargain table or outlet store, it can drop another 20 percent or more.
In my view, 50 percent of retail should be the absolute highest price paid for any currently available tool. Once production ceases, the value should hold as long as the condition and appearance remain. As described above, some defects really diminish the value of a used tool, personal markings apparently being the most egregious.
Other factors that influence the desirability/value of a used tool include:
Objective criteria:
Damage: defects, wear, repairs, rust, flaked chrome/finish, appearance.
Functional issues: present usefulness, performance, quality/strength.
Immediacy of need, intended use, ability to obtain a substitute tool
Subjective criteria:
Design, appearance, Snap-on handle color

Uniqueness, scarcity, collector value
Age, history, provenance
External factors:
Shipping and transaction costs
repair/restoration
warranty/replacement
storage, display
I personally don't pay more than about 33 percent of retail price for any used tool, and usually a lot less: 25 percent and below, sometimes WAY BELOW. But I am not shopping for a specific tool, and don't really have a particular need. I am simply trying to acquire what I don't already have too many of at a price that will later allow me to sell it to the next guy without losing money.
Someone else might stumble across a rarely encountered tool in good condition that is no longer made, and is unavailable at retail. The tool will help complete a collection. He might be willing to pay 75 percent of former retail, or much more. That's OK, too.
I have bought newer Wilton vises in near new condition for about 15 percent of retail price, and paid probably 1,000 percent of original price for 100-year-old vises that were in fair to good condition.
Despite all the numbers and formulas, it boils down to "desirability" and "timing."