This idea would in principle decrease the number of trades between people, however, trading is at an all time low recently (compared to when 3.0 just started). The only thing you used to see in chat was people either trying to sell or buy something. Now it's usually just pvp or random nonsense.
I guess what I'm saying is that because there is so little trading between players at the moment, implementing this would not have a very negative effect on player to player trading. It might even stimulate it when people see the prices in the shops and try to get better deals from online players.
As for scamming, use common sense just like player to player trade. If someone is offering a stack of diamond blocks for 1k, it's probably a scam. If you see a random button with no sign that wants 10k to be pushed, don't push it. If you see a button selling diamonds for 30 rc, be skeptical.
I do foresee an issue with iron doors. Often, due to the lag, it is very hard to get through a door after pushing a button, I'm sure many people have experienced this. This means that people may be forced to pay multiple times before they manage to get through. Ofcourse whoever sets up the redstone circuit for the door could just add some delays that keep the door open longer but not everyone will bother to do this.
mitchie151 wrote: It has also been previously stated in the thread that empty dispensers would not charge you.
I have a question about this (for Intelli I guess, or anyone who knows the minecraft API well). How does the button know it's connected to a dispenser?
As stated in the OP the button could be used to anything like doors, railways, even donations. So how does the system know whether the button is connected to an empty dispenser or a door? What if it is connected to both? How do you know what you are paying for?
I think solving the empty dispenser issue will be quite tricky.
You could solve it using redstone circuits maybe. If you have the item you are selling drop onto a wooden pressure plate first. Only if an item has dropped onto the pressure plate the button will become availabe again (otherwise you just have a piston move infront of it or something). This does mean that you will scam the person who presses the button after the last item has been dropped.
Wow, I'm on a roll, I'm going to stop now.