I have in the past, but like you say it's rarely worth the shipping especially for the 3.5" ones. Instead what I do to collect lots of drives, is I offer free digital wiping service of old hard drives, just to keep the salvage of the hardware. From just a few good size companies I get hundreds of old drives. Many have bad sectors or have failed which is why they were replaced, but they are almost always still good for data recovery donor heads or at least a pcb. The ones I get in that can't be digitally wiped, I just destroy the platters and keep the PCB.