I will say the shop should not have pushed these boots if they were feeling even the slightest bit uncomfortable to begin with, either the employee didn't know about boots fitment or they were just trying to make a sale.
I ride Burton rulers, when I got to my local shop I tried on other Burtons, 32's and K2's but even the higher end models there didn't fit me right at all. We all have different feet, so the most important is just finding the right boot. Yes a stiff boot will break in, my Rulers seemed stiff out of the box but have mellowed out quite a bit. Basically if the foot to boot fit isn't right, the boots breaking in really won't help much. Even different boots within the same company will fit differently.
If you are going to sell them, don't break them in. Let the new owner of them break them in so the boot will break in to their feet instead of yours. Could help as a selling point.
The shop won't accept returns even with a receipt? I find that pretty low on their part, obviously people are trying on boots there, so whats the difference if someone took them home and wore them a little or not. Sorry you have to deal with a shop like that, just don't brand-hate due to a specific boot. I tried on the freestyle Burtons too and it felt horrible, but the rulers from the same year felt perfect, just do as much trying on as possible when you get a new pair.