If you have a gunshop or gunsmith shop, then it is simple. Put a sign under your shop sign that has the words “We Can Dispose of Your Old Ammunition”. It seems that people find in their antic, garage, or other ammo that may have been there for many years. Now legitimately some ammunition need to be disposed because of corrosion, or other reasons. However, theoretically portion could be that valuable collector’s item that you are looking for. You would be doing a valuable public service in addition to occasionally buying some real jewels.

A interesting concept, but could be problematic w/ any API/incendiary/tracer rounds.

Example: in Kalifornia, ordinary ammo (ball, AP, sporting JSP, JHP, etc) is fine for most folks to transport.
Beware if otherwise, as the Kalif V.C & P.C. have section(s) prohibiting ordinary citizens from transport of live tracer, API, APIT or straight incendiary ammo upon any public road. The only exemptions here: LEO & active/reserve mil.

A friend worked at a large gun shop in Houston, said people came in almost daily with old ammo to dispose of.