When I sell locally online, why do most people never show up or if they do, it's way after the agreed time?

The woman who stood you up wasn't serious about purchasing your items.  She probably just wanted to have a look, and since she was not serious, she didn't care to let you know she wasn't coming.  I've sold on both, and it's really hard to sell on craigs and FB, mainly because people don't want to make the trip to see the items.  A way to eliminate the no-shows, and potentially get more people, is to let people know a day and time you'll have the items available for purchase. For example, you can say that you'll be at the so & so parking lot between the hours of 10-12 on Saturday or Sunday.  Or, you can just have a yard sale, which I personally think would be better for you.  If there's a flea market in your area, you can also set up your things there. Otherwise, waiting on each item can be a real pain, as are the no-shows, and it will probably work out much better if you just have all your things in once place to sell.  

Do not wait for them. I would give them  20 minutes unless they contact you that they will be late. 

Find another way fo selling your stuff.

They should contact me and let me know they aren't coming or will be delayed. No excuse for this rude behavior. One woman stood me up 5 times and always with some lame excuse. I finally blocked her from responding.

They are probably busy and agreed to meet you during a break in their schedule but then things didn't work out how they thought it would