how do you say that in Greek?

Depends on what you mean by that? The phrase "beware of Greeks" is Englished from the Aeneid and in Latin is: Quidquid id est, timeo Danaos et dona ferentes. A synonym of caveo 'to guard' is the Greek phulasso. The Latin is a hortatory subjunctive (now what's the Gk form?). Gk agoraste:s (< agora 'market; assembly') is a good word for emptor. Oh, well: phulasse agoraste:s. (Though I doubt it';s correct; I'm not sure you can use the 3PS like in Latin.)