Mercado Sebastián Escobar is Tapachula at full volume: fruit, meat, sweets, clothing, electronics, lunch counters, and the everyday buying and selling that keeps the city moving. It is more useful than picturesque, but that is part of its value.
Go in the morning if you want the market at its busiest. It is a good place to look for tropical fruit, local snacks, and the practical mix of Spanish and Mayan languages that marks this part of Chiapas.