When you want to know what the correct text of a card is, just look it up on Gatherer. A lot of cards like Merrow Commerce were printed before significant rule changes were made, and while the official text on these cards have been updated to reflect this, the cards may not have been printed since the change. Should Merrow Commerce be printed again it would say "at the beginning of your end step." It's similar to the difference between "play" and "cast." Whenever an old card says "play," it's understood to mean "cast."
In the case of Merrow Levitator , it has the triggered ability that untaps your merfolk at the begging of the end step and within the endstep, there is a round of priority where you and your opponent can do things (activate abilities, cast instants) before the turn actually ends.
July 11, 2014 12:32 a.m.
In the old days, back in prehistory, the end step was the end step, and so cards were fine with less specific language like "at the end of your turn." This simplicity left some measure of ambiguity about how to treat end of turn effects, so in the modern era, the ending phase is composed of two steps - the end step and the cleanup step.
During the end step, triggered abilities that trigger at the beginning of your end step or at the end of your turn are put on the stack. Merrow Commerce 's triggered ability goes here, along with any other triggered ability. In this way, both phrases mean the same thing. Players receive priority as normal - starting with "you" (since it's "your" turn). After the stack is handled, your turn moves along to the cleanup step, where damage is removed, and etc.
Oracle text standardizes the old language ("at the end of your turn") on printed cards with the modern language ("at the beginning of your end step") in an attempt to clarify these things, believe it or not.