Hilchot Supermarket.

I recently posted about the mathematics of standing in line in the supermarket. There is something I hate more though, and which seems to be an Israeli characteristic:

1. People who put their Supermarket cart at the lineup, and then go to fetch more items. I'm not talking of someone who suddenly remembered they forgot 1 item and makes a dash for it. I'm talking of people who intentionally leave their cart to "Keep a spot" and then go to collect a myriad of items of their list. This is really a pet peeve of mine. This seems to occur almost every time I'm at the supermarket.

2. People who show up - ask "who is the last guy in line" and then tell him to keep their spot, and leave. I hate it when I've lined up, and suddenly discover that someone was in front of me without me knowing.

To be honest, I don't think you need to find a halachic principal to show why these two things are not allowed. Good old "Derech Eretz" should really be enough. However this is Israel we are talking about. I was surprised to find that I couldn't really find any sources for either case – surely I'm not the only one who thinks this is inappropriate behavior?

I think at least the second case should be considered a "Gezel Zman" - Had I known there were other people in the line I probably would not have joined the line.  Time in Halacha = Money. 

This could be inferred from the (32b) which deals with two boats arriving at a section of the river, where only one can pass. The boat that passes first, has to compensate the other for the time it had to wait. This would be even more obvious in waiting in line at the supermarket - where standing in line clearly gives you a right over whomever is behind you.  

The first case is slightly harder to pin down. Theoretically, as long as the person isn't holding up the line while he goes to find more items, he can claim that I didn't loose any time. However I would answer that I would have been before him in line, had he finished collecting all the items he needed before he left his cart. Therefore this is another case of Gezel Zman. Furthermore his behavior is against the Minhag of the place (תקנות סוחרים).  

I would appreciate anyone who can help me find better sources for what seems so obvious to me.