Why do people say 慢点儿阿("man dianer a") when you leave their shop? When you leave a shop, you often hear the shopkeeper saying something like "man dianer a", which sounds like 慢点儿阿 (something parents say to their kids to get them to walk more slowly [even if they never listen]).

It's quite possible I'm mishearing what they're saying.  It really is an everyday thing to say, and I've heard it in lots of different and unrelated shops.
慢走 is a polite thing to say usually used for someone who is leaving, has two implicit meaning:


I don't want you to leave, so please leave slowly, so I can stay a little longer with you.
Don't hurry, take care.


慢点儿 has the same meaning, but not as formal as 慢走, and sounds more affable.