You should have posted this to the bug section... why you ask? Because it's a problem that many players are experiencing... and there's no way, at least now, to solve this problem. (Try closing the shop when you see that too many people are entering. That should get them to go away... for a while...)
Based on my gaming experience. Every time you meet a customer who wants a shield. If you sell something to him, your customers will add one more person. Refuse to serve customers who want shields would be better.
