Mexican Spanish has a verb for it. The dictionary of the Royal Spanish Academy marks amarchantarse as Mexican and defines it as “hacerse cliente habitual de un comerciante” (to become the regular customer of a merchant). The regular customer is the marchante. Many market vendors tease their colleagues roughly, the magazine Chilango wrote, “pero respetan al marchante” (but they respect the regular customer).
The relationship has a reward. The pilón is a small extra on top of the purchase. Matador Network in Spanish describes it as the extra that customers earn “por nuestra fidelidad” (through our loyalty).