Compramos
a
diecisiete
y
vendemos
a
dieciocho.
kohm-PRAH-mohs ah dee-eh-see-see-EH-teh ee behn-DEH-mohs ah dee-eh-see-OH-choh
We buy at seventeen and sell at eighteen.
Exchange houses have two rates: 'compra' (buy — what they pay you) and 'venta' (sell — what they charge you). You want the 'compra' rate to be as high as possible.