What is a sales hat trick?

The term "hat trick" in general refers to three of ... something. The usual context is a sporting event, where a player scores three points (modified somewhat for higher-scoring games; no one calls a field goal in football a "hat trick", but they might call it a hat trick if the same kicker scored two other field goals in the same game).

There isn't so far as I know any hard and fast definition of "sales hat trick", but presumably it refers to three sales. Again, this is unlikely to be used for a cashier at the grocery store serving three customers, but it might be appropriate for a direct salesperson at a large firm landing three significant accounts.

Incidentally, no one really knows why it's called a "hat trick". There are a couple of theories, but they're far too literal for them to be believable ("some sports club awarded players who achieved it a special hat" and "players who did this passed their hat around for coins" are among the more commonly heard). I personally think it's more likely a reference to a stage magician producing a rabbit (or something) out of an apparently empty hat, a similarly "amazing" feat (that actually happens all the time, but in fairness not everyone can do it).