Well for one it undermines the currency by making my dollar worth something different from your dollar, makes it impossible for individuals to make informed choices, and makes it so that there's no reason to try to be more productive/chase some higher paying endeavor since it will be extracted for your basic necessities anyway and you'll end up in the same position. So now labor price signals fail.

We already see the results of this in the US in the medical and educational systems, where planning is nearly impossible because the price is hidden until purchase (or even afterward) and the other side is trying to price it at "whatever you have". Unsurprisingly, both of those have absurd debt loads and are causing society-wide discontent/destabilization.

Now imagine that grocers can individually price to you, and you find out what you owe (and whether you need to put some items back) at the register, and this information about you is available to be purchased by anyone so all available grocers have that same knowledge. What are you going to do?