I would accept an untextured rectangle as a shop, as long as you take care for the prices.
The stuff bought in shop should drop similar as the items bought in other worlds in front of the shop.
shop-idea:
flying chariot (ballon, blimp, airship) pulled by dragons or imps- somehow as Santa and his sleigh?-
player could have to place a signal (pull up a flag?) so the trader will stop in our world the next day only if signal set.
Perhaps the signal has to be crafted so the shop is not available in the beginning.
It could also be a platform (airport, parking lot) to craft for the shop to land on.
The platform/signal could be target of enemy-attacks.
No parking-lot/no signal? no shop...