I think the forum rules need to be amended, because "I'll take it" should not automatically mean the seller must hold it until whenever the seller feels like meeting, even if that's a few days out. I would never hold on to something for multiple days for a possible buyer (who could absolutely still flake) when I know I will have other willing buyers in the meantime.
If you aren't able to meet the same day you post "I'll take it", you should have to post something like "I'll take it if you can hold it for me until X day". Totally unreasonable to expect someone to hold something for multiple days and potentially turn away multiple willing buyers. In this case, a Glock 34 for $450, that's exactly what would probably happen.
Edit: I'm not sure if you originally told him you wanted to meet Friday, because post #9 makes it sound like that's what happened initially. If we're talking same day, that's obviously a little different.
Still, simply posting "I'll take it" by itself should not be an automatic, guaranteed hold with no ifs, ands or buts. You should have to communicate and make a plan first before you're automatically committed. As we've seen, too many people post I'll take it or otherwise express interest, only to back out or flake.
When I sell stuff through other selling mediums, it always sells to the first serious buyer who shows up with cash. I don't hold anything until we've made a firm plan with a specific day/time. Holding something for an unspecified amount of time based simply on "I'll take it" just is not reasonable. If every person was reasonable, it would be.. but not every person is reasonable.