If I was in a store and wanted to know if they had apples would I say to the clerk, 'tiene manzanas' or 'tienen manzanas'? I suspect the former although I'm not asking if he personnally has apples but rather does the store where he is working has them. I suppose in the southern United states, one might ask 'do you all have apples'.
To clarify a little, I'm more interested in the conjugate than the specific question. That is, when asking a representative of a business what the business has for sale or what services it offers do you use the singular or plural conjugate?