Agreed. Anything we identify as good, others may do the same leading to a glut of those items. It's probably better to have spare supplies of the things you want for yourself, than to use the space/money/weight for something you hope others will want.

A lot also depends on the nature of the disaster. Spare batteries might be invaluable, or they might be useless, for example.

I've done that, more or less. Not for survival, or for trade, though. It was an adventure holiday in Venezuela, with a tour guide but a "dig your own toilet" affair. We stays a few nights with a remote tribe, and were encouraged to bring gifts for them. I brought sewing needles, and they were one of the items most valued. Make-up was also appreciated.