Abarim Publications' online Biblical Hebrew Dictionary
There are two separate roots of the form אגר (agar), which at first glance don't seem to have anything to do with each other, but see the Greek verb αγειρω (ageiro), meaning to gather, from whence comes the noun αγορα (agora), meaning market, the place where both food and labor was traded.
The verb אגר (agar I) means to gather (food). It occurs three times, in Deuteronomy 28:39, Proverbs 6:8 and 10:5. A more common Hebrew verb meaning to gather food is ארה ('ara), hence the name Ari. Another verb that has somewhat the same meaning is קהל (qahal), hence the name Qoheleth, which is the original name of the kindred book Ecclesiastes.
The unused root אגר (agar II) exists in cognate languages where it means to pay or hire. A more common Hebrew equivalent is the verb שכר (sakar — see the name Issachar). But we know that this root must have existed in Hebrew because one derivative survives, namely the feminine noun אגורה (agora), meaning payment (1 Samuel 2:36 only). Another noun אגרת (iggeret), meaning letter (Nehemiah 6:5) is probably a loan word from Akkadian, and looks like it has to do with our verb by accident.