Because you replied to it and now people are talking. When someone posts in a thread, it moves to the top. You would have found the thread with a search or a few pages deep and buried with no one looking at it. When you replied, it moves to the top and every time someone posts it goes back up again.
This is an old post, and has been happening a lot because people do a google search and find a 2 year old post about handguns or sales and reply where can I get this. Then everyone sees the post and thinks its new and jumps all over it. People get frustrated and then someone writes a post like this one explaining why this happens and eventually people stop posting and it dies away again.
Take someone shooting - Encourage someone to get a Pal/ RPal
Just happened to me the other day with a mag sale.
Someone ressourected an old thread and I took the bait and jumped on it right away.
I would venture to guess, that most people are not in the habit of checking the tiny date in the left upper corner, once they see a great deal posted on the top of the page..
"Gun control" is a feel good term, which is designed to disquise the globalist disarmament efforts under the pretense of fighting crime.