Through the years I've bought domains for hobby projects and such that ultimately I'll let expire. Usually after receiving tons of emails, such as you've described, it's followed by a "we now own your domain, pay up" threat.

I've even had firms purchase other extensions for my domain (particularly a .org, .net, or .co), then email me asking to buy it from them. I usually respond and tell them I'm selling the .com domain myself and they can purchase it directly from me for half of the price they're offering to sell me the other extensions.

No one has ever taken me up on my offer.