Wow.
So, back in the 4th edition days, I ran the D&D Encounters program at my FLGS. Wizards would send physical copies of a module to a hobby shop for free, including poster maps and, sometimes, supplementary material like NPC or item cards or something. The shop would get DMs to run a part of the module every week for 2 hours on Wednesday evenings. WotC got free press, the hobby shop got foot traffic, and DMs got a module for free; everybody won. My short reviews of the ones I ran are 
here.
Eventually they spun off a Lair Assault series as well. These were much shorter booklets, 16 to 24 pages, covering a single tactical encounter; each one came with a two-sided poster map, tactical cards for the DM showing starting positions, etc. The brief reviews for the ones I ran (all of them) are 
here.
Obviously, the modules from Encounters had a lot more meat than the Lair Assaults. I ended up with extras for most of everything, and occasionally someone reaches out to me to sell them one.
I just checked. Most of the Encounters modules sell for $25 to $50 which is, I guess, a fair price. I wouldn't pay that for most of them, and would think that price a bargain for 
Murder in Baldur's Gate or 
Legacy of the Crystal Shard, for instance.
I just looked up the Lair Assault modules, and they're going for, frankly, 
ridiculous prices. 
Absurd. All of them are going for at least $40. 
Forge of the Dawn Titan is a 16-page booklet. It's selling for between $150 and $200. 
Into the Pit of Madness is a 22-page booklet. It's selling for between $80 and $100. Those last two are 
by far not the best of the lot.
Is it collectors?... Maybe I need to hold onto my leftovers for another decade or so, and see what they're selling for then...