As satisfying as that may have been, the marketing result - not so much, I think. Not surprisingly. Such incoming mail goes to very low level labor, tasked with processing respective return slips. Anything else found in the envelopes - into the trash bin it went, without a second glance, I suspect.
In reply to OP: I think you're on the right track. I have not tried this yet, but getting ready to market my brandables by snail mail. No postcards, however. Attention to detail and professionally worded business letters. Sent from a COMPANY, not an individual. In quality envelopes, addressed by name and title to top management, no less. Given the $XXX-XXXX price range of the assets for sale, mailing cost is a no-brainer. I'm old date: you gotta shell out a little money, to make more money! Processing, using a mail-merge app, not much more effort than sending out spam... er, emails
Actually, factoring in email bounce and other issues, this may well turn out to be less hassle.
Thought of faxing the letters, too, but still on the fence about that idea for a number of reasons...