do people list items for sale, then when they have someone that can buy them, they sell to someone else, even though someone had already committed to certain items?

i've had this happen a couple of times recently, and i was supposed to buy some bike stuff this weekend (couldnt during the week due to distance and me working), but was going to pick them up today or tomorrow.....so i get online to check today for contact info to go pick up the items....and they were sold.....

you also have the same thing when trying to sell something, someone shows an interest in something, asks for pictures and price etc....then you dont hear from them again.....