Occasionally I get people in my booth at shows asking for a discount. It is always an uncomfortable moment... as I don't want to offend if they truly do not know any better. Depending on how the tone of the discount inquiry is presented varies my response. I often say yes there is a discount for frequent customers. That way they understand there is potential , but since there is no established seller/customer relationship no such discount. But I leave he door open for the start of a good/frequent customer relationship.
I do have many many repeat customers and often give some way recognizing their loyalty either through an unexpected discount or additional add on at no cost.
Someone once gave me good advise .. its takes only seconds to loose a customer , and years to build a loyal customer.