Since the question presupposes that there's a good reason for me taking pictures of the inside of someone's shop, convincing a shop owner to let me do it would be relatively simple.
There's a couple of reasons I've done this in the past. One is, there's a product my wife wants me to buy, but I'm not sure which it is, so I take pictures and send it to her via text. The other, as a newspaper owner, I wanted to highlight a business and thought it would be good to show the interior of their store.
In either case, the photos benefit the store owner, so it's a simple matter of announcing my intentions from the start. I think as long as someone is upfront and honest with what they want to do, and it's something that actually benefits the store owner, it won't take much to convince them.
However, sometimes the store owner/manager/employee still won't allow it because of corporate policy.
A friend of mine and I wanted to put together a website/mobile app called The Hungry Traveler. The idea was, we would have a comprehensive site/app with information and photos regarding all the places to eat within a city. We hoped to get businesses to pay for this eventually, but the idea was to get it out there, gather a following, and then take care of the revenue side.
Well, we didn't get very far, because McDonald's corporate policy didn't allow for any photos taken by anyone other than their own official photographers. The pictures they produce, of course, are very polished and show things in the best possible light. We weren't trying to make things look bad, but we did want to show original, untouched photos—in other words, show this particular McDonald's as it was.
We found out after that McDonald's wasn't the only place with such a policy. I guess we could have lowered our standards and used press kit photos, but it seemed to take away from the whole reason for doing it, so we were basically stopped in our tracks. We could have just gone with local people, who probably would have been just fine with what we were doing, but that would have shot the idea of having every place to eat show up on our site/app.
Branding, image and security are three pretty important things to shop owners, regardless, so even if you do have good and proper intent, you need to make sure you can deliver decent pictures of the places you photograph, or you will wind up in trouble anyway.