Yeah, if you are buying new parts or non-core parts, don't just toss the old ones. At least keep them around in case someone needs to fix a cracked case or gear to rebuild theirs. I've also given cores to people to return when they buy something and they don't have a core to return, like when you change out a factory Denso 40A alternator for a GM CS105. A guy might want to keep his still good factory alternator as backup or in case he sells the truck, so you just shove whatever core you got sitting around in the box and most of the time the counter guy couldn't care less if it's not exactly the same thing.
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"They say the test of literary power is whether a man can write an inscription. I say, 'Can he name a kitten?'" -- Samuel Butler