what to say at a mercado or tianguis

Pointing is not a failure, it is the local method

The fear at a market stall is almost always vocabulary. You know what you want, you do not know the word, and you stall trying to retrieve it. Watch how Mexicans shop and you will notice they point constantly. De este, por favor is a complete, normal, adult sentence at a tianguis. Nobody is grading your produce nouns.

The units matter more than the names. Most things are sold by the kilo, and medio kilo is the default small purchase. Un cuarto is 250 grams. If you want a number of pieces instead, say the number and point. And ¿cuánto le debo? is the phrase for asking the total once you are already buying - more common at a stall than ¿cuánto cuesta?

One social rule is worth knowing. At a tianguis you can look without buying, but walking away in silence after asking a price reads as cold. Ahorita regreso is the polite exit. It means later, it is understood as probably not, and everyone is fine with it.