Roadster Shop,CHC,Ring Bros, DSE, Ironworks,CAR, Speedtech,Ridetech, etc what do

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And all the many,many other busy I shops I can not fit in the subject line!
I always wonder what all the shops & t.v shows for that matter, do with all the take off parts ? it seems like alot of stuff is still good or useful maybe to someone else, now I know some stuff is just plain worn out and I would think goes into the metal recycle bin, I am wondering about good useful parts or parts that maybe a person could use to do a stock restoration, also motors maybe that were recently new and now the customer is doing an LS conversion.
I will use the Roadster shop as an example because they have multiple projects going on and I see some pics of some car's that they post as the starter project and its like wow that thing looks good the way it is and they are doing a full rebuild, what do you do with what they call "old stuff" ?
Do shops eBay the stuff? do the customer's come and take it back? or does it get recycled at least and how do you keep track of all that stuff in a busy shop? specially if the customer wants to know what was sold or taken off
Thanks for answering my curiousity