If I were giving away a POS swing set, I'd probably be giving it away knowing that it's either going to be work for me and it's an eye sore/burden.
Did you communicate with the family that showed up and set clear expectations? Did you tell them to bring tools, that they would have to disassemble it and move it on their own? Did you tell them you were working on a bid and set a time deadline?
If you did, then you had plenty of opportunities to call off the move last night. If you didn't set these expectations, one advice I'd give you is never assume or have expectations when selling something online/giving it away. I always try and assume they don't know, or won't do, or have a different expectation unless I make it clear.
I just gave away a bed last week. I made sure they knew a time frame and if they weren't there by then, we'd be leaving for dinner. I told them I took it apart but would not be able to carry it because of my back. I told them it was heavy and they needed at least two, strong people to carry it. I told them to bring a truck. Lol Ultimately, it was free, but they did a lot of work I would have done if they didn't get it. I'd imagine cutting up your swing set, burning it and dumping wouldn't have been fun either. On the positive side, just be glad it's gone and that **** isn't in your yard anymore.
You have a big heart, dude. While you may hate people, you're going to help people when you can, or put others first - even if you bah humbug about it afterwards.
I remember when my brother died, you and Trout called me to check in on me and to see if there was anything you could do for me, even though we hardly knew eachother. That's something I'll never forget. I'm not sure how grateful the people were last night for your help and swingset give-a-way (some people are oblivious) but I can guarantee you made some kid happy - up until he/she breaks their arm on that damn thing.
Lastly, this is water. This is water.