maido = each time (lit.) ari = short for "arigato" So this is a vulgar expression of "thanks (for coming back and shopping with us)," typically used by greengrocers, merchants at market stalls, pubkeepers, etc. Here, "maido" may imply recognition as a regular customer and a sense of intimacy. p.s. "maido arigato gozaimasu", which means the same, sounds much politer and we can hear this almost every where.

"maidoari" is abbreviation of "maido arigatou gozaimasu" you can hear this "maidoari" at vegetable store, fish store, long-established eating place(perhaps), and so on.I think it is usually used at market place. on the other hand, in the case of "maido arigatou gozaimasu", you can hear at restaurant, department store, and so on. the former is informal, and the latter is formal.