Welcome to the Forum.. That doesn't sound like happy purring to me. It sounds like he's a frightened and unhappy boy. Piggies are prey animals and need time to get used to their new surroundings. You need to put yourself in his position. He has been ripped away from his family and put in a shop, just as he was getting used to that he has been taken away and put somewhere strange with new noises and smells and a huge giant (you) with hands larger than he is who keeps touching him when all he wants to do is hide somewhere quiet. Wouldn't you be scared in that situation? I know I would! You know you aren't going to hurt your new little boy but he doesn't know that - you have to teach him that and that takes time, patience and a lot of love. He is running on prey instincts at the moment. Keeping still in the hope that you will ignore him and he can run away! Does he have a friend? Piggies are not wired to be alone.
Here are some guides for you to read to help your piggy to settle in.
Getting Started - New Owners' Most Helpful Guides