Man, I wish I would've seen this thread a couple months ago... I changed strings on 4 of my basses (three 5'ers and my 6-string). I'll definitely start saving my old sets for sure. I've currently only got a couple sets that I'll definitely save for this (one is a set of practically new Carvin/LaBella flats that I swapped for nickels 3 days after getting the bass) and I'm about to change strings on my two main players. I know it's not much, but it's something.
I also mentioned it to Dan Basica, owner of Link Removed and sent him your link. He said he emailed his staff because they do a lot of string changes and never really know what to do with the old strings, so they're going to save too.
I know they're not strings, but I've got a pretty big collection of guitar picks I could send your way too, if you want them. I've got a guitarist friend who goes to NAMM every year, and he handed me a tin with a bunch... maybe 150 (random variety) at a guess?
I think what you're doing is awesome... I'm blessed to have nice gear and can change strings pretty cheaply almost any time I want. If I can help another player put that 4th string back on their bass, man... a few bucks in shipping is WORTH IT.
5sg.