I understand the tactic and the logic behind it for a variety of reasons. I will try to explain my thoughts on it.
First, of course it is a tactic to get people to "pull the trigger" who might still be on the fence. When someone has posted something for a while and it isn't moving and you might be interested, there is no hurry because maybe you don't need it or maybe you want to wait until the last moment to drive the price down, or whatever. So the seller says "this I the last chance, and if nobody buys it, it goes in the trash". At that point if anyone IS interested, they will at least make an offer.
Second, it is not worth the time to sell something and ship it if I am not making something for my time. If I can't make a certain amount of money, I would sooner trash it and have the free time for other things. Perhaps I don't have the space, perhaps I don't have the free time to deal with shipping, or perhaps I am just so sick of seeing it sitting around that I just want to wash my hands of it. Whatever the reason, I have thrown out stuff that I couldn't sell because I didn't want to waste my time trying to give it away.
I had a Diablosport tuner for my Mustang that I bought used for $250. These are tied to the VIN of the car and to use it on another car you have to restore the car to the previous program before you "unlock" the device. Long story short, I got a second programmer with my supercharger and didn't restore and unlock this first one before loading the new program. Then I had the second programmer reset with a new program and lost the original, so I couldn't unlock the Diablosport. The manufacturer charges $150 to reset it, and I tried to argue that I had the original ECU and VIN but I had lost the original program, but they only cared about their $150. The used value of these was a pretty solid $225-250 at the time, and I didn't want the hassle of putting up my own money to reset it and sit on it until it sold for full price, so I simply advertised it at $75 and explained that it would have to go back to the manufacturer for a reset and it would cost $150. I guess nobody else wanted the hassle of sending it in either, so it never sold. Since I couldn't sell it for what it was worth to ME, I took it out to the range and blew it up and then posted the video. I took great satisfaction in doing so. Sure, I could have given it away, but the hassle of taking it to a shipping outfit, packaging it up, getting money transferred, etc just wasn't worth it for no money. That time, to me, was worth at least $40, and any less was not worth my time. BUT, I would have taken $40 or $50 for it. Nobody offered though so I disposed of it in dramatic fashion and got SOMETHING out of it.
I am the same way with garage sales. While the garage sale is on I am there to make a few dollars for my time, and after the garage sale is over I am not going to waste a single second trying to give it away. Now, if I were just packing up a garage sale and someone dropped by and wasn't interested enough to pay for something and was walking away and I was ready to haul it out to the dumpster, I might stop them and ask if they want anything for free. But I spend X amount of time selling, and then I am done with it and don't want it any more. If I gave it away to one guy, then another might show up two minutes later who would have bought it. So that is literally a "buy it or it goes in the trash" scenario. Once I decided to garage sale it, I no longer wanted it in my house. I am not a hoarder so it is not going to sit until it sells or whatever.. I am going to throw it away and feel good about it. I will always give clothes to charity, but there are plenty of things you can't give to Goodwill (including CRT TV's)..
So yeah, I completely understand why someone would say "$100 or I dump it today". Frankly if you are that interested in the product being dumped, you should make an offer that is worth the person's time to deal with.