Based on the facts as you have stated them, it appears that the vendor has indeed defamed you.
According to Georgia law, and in particular O.C.G.A. § 51-5-4:
(a) Slander or oral defamation consists in:
(1) Imputing to another a crime punishable by law;
(2) Charging a person with having some contagious disorder or with being guilty of some debasing act which may exclude him from society;
(3) Making charges against another in reference to his trade, office, or profession, calculated to injure him therein; or
(4) Uttering any disparaging words productive of special damage which flows naturally therefrom.
(b) In the situation described in paragraph (4) of subsection (a) of this Code section, special damage is essential to support an action; in the situations described in paragraphs (1) through (3) of subsection (a) of this Code section, damage is inferred.
With that in mind, it appears that by calling you a thief, the vendor has imputed "to another a crime punishable by law," and accordingly, engaged in conduct defined in O.C.G.A. § 51-5-4 (a)(1).
If you do decide to move forward with a lawsuit, you should consider a few factors, such as whether it is worth a lawsuit in terms of a cost-beneift analysis, and any defenses the vendor may have.
Feel free to contact me if you want to discuss the matter.
DISCLAIMER: Although I am an attorney, absent a signed retention and engagement letter, I am not your attorney. There are no exceptions to this rule. Moreover, you shall not rely on the information I am providing you, as it is only for your general knowledge and educational purposes. Consider it a good first step in your knowledge acquisition, but not legal advice. Indeed, my analysis is based on the extremely limited facts you have given me, and new facts could substantially alter any answer I give.
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