à = at I find "acheter au vendeur" to be correct ("I bought an apple of the seller" if you will) but equally "acheter du vendeur" ("bought from") suits me. "je la lui ai acheté" is quite formal. Your sentence mean "I bought it from him" whereas "je l'ai acheté du vendeur" suggested by Zayer get rid of one pronoun, so it's easier to read (even for a native French speaker - we don't particularily like the use of more than one pronoun per sentence.

I think you would rather use the preposition "à" when you are buying something at an actual shop (something implying it is a building/place), so in cases like "à la boulangerie", "au supermarché". "au kiosque" I am not really sure about any set rules for these prepositions, but as far as I know when the vendor is a person would most often use "acheter qc. chez/de qn.". You could also "chez" in this case (at least I have heard and read it that way), but I don't think it is used as frequently as "de". Bonne chance ^_^

on dit j'ai acheté une pomme du vendeur et non pas au vendeur! when you use pronouns il vaut mieux dire : je l'ai achetée du vendeur.

C'était un exemple seulement titop :)

tres bien Craig. la pomme du vendeur si c'était la sienne personnelle or c'était celle du magasin donc ce n'est pas la pomme du vendeur. Donc je suis d'accord avec Alexandra. Craig "il met des pronoms partout" a encore frappé. :))