"Yup...$3.00 per each times 25 in a box = $75.00 per box. Shoot them up, what's $75.00 anyway. If one is concerned about online fees and shipping charges, Craigs List has no listing fees and you can specify in-person pick-up. So, go ahead and shoot them."
You're assuming many things in that statement, and all of them are what I call PERFECT assumptions (that is, everything will come together perfectly and $$$$$!!!!! Right. That's not how it works 99.99999999% of the time.)
1. That you'll find someone on Craig's List who is interested in buying "collector" shotshells.
2. That they will be interested in ALL of the shotshells (otherwise, see no. 4).
3. That they would be willing to give you $3 a shell (which, remember, is the price on a COLLECTOR site aimed AT collectors).
4. That you'll actually be able to sell off all of the shells within a reasonable period of time... say, 10 years.
That's a damned lot of assumptions, and you've got absolutely nothing to back up those assumptions other than a simple, and flawed, speculative formula -- If old paper hull shot shell, then $$$$$$!!!!!
A South Park fan would call that the "underpants" formula from the episode where gnomes steal underwear based on their business formula of 1, collect underpants; 2, ???; 3, Profit!
I've been collecting cartridges, including shot shells, for the better part of 40 years. It's not at all uncommon to go to gun shows and see many dealers with loose paper and plastic shells on their tables, most of which are priced at 25 CENTS or less.
Often, modern plastic shells are priced far higher than the old paper hulls because, well, plastic is assumed to be still functional.
Maybe you're missing a perfect business opportunity? Go to a gun show, buy up a bunch of cheap paper hull shot shells, and then sell them for $3 a pop on Craigslist...
I'll say it again...
Just because something is somewhat old doesn't mean that it's an instant antique or that it has value (real or imagined) to someone else.
To think otherwise is unrealistic and, well, silly.