You can click on the cash register or the item to buy it. I think the item needs to be on a ss mat or shelf or pedestal. Even items hanging from the ceiling are buyable. In game I find clicking on the items easier as the cash register lists everything on the lot that counts as being in contact with a savvy seller thing. Even windows and. Curtains. The register lists everything. For example, If you have 10 of something it is listed 10 times.
I find the register attendants wander off too. I only hire one if I decide I want them in one of my videos. I rarely put these stores in my big game worlds now though due to memory usage. I have hundreds of sims in most of my worlds in that game. Over 800 sims in total. And that keeps increasing.