The chance of finding them are the same chance you have at finding just about anything in a locker.
That may be true if looking for a specific item; say I was LOOKING for a particular watch I either wanted for myself or that I knew sold well. Chances are I wouldn't get it in 100 lockers I bought.
On the other hand if I feel that TOOLS in general do well, I am more likely to find some tools that will sell, though again the BEST tools or the BEST sellers aren't going to come up that often.
That's why as I think you said, the bread and butter items pay the way most of the time and the really good stuff is just bonus time.
As another example, generally the toys I get aren't that good, but, perhaps like you, I have learned that American Girl dolls and the wardrobes they can travel in do quite well for the most part.
Depeding on the socio-economic status of the former lkr owner, most of the lkrs have just average toys to below average, but once in a while I find a lkr with a lot of good quality toys and the orignal boxes they came in. In quantity those can pay off just as a dozen of your mugs do.
A toy train? Thomas the Tank Engine...OK. Lionel from 1938.....almost as good as a small grain of gold.