It is very easy to arrive to Mercabarna. Here you have a map so you don’t lose.
From Barcelona:
Route 109: From the Plaça de Sants or the Plaça d’Espanya.
Route 21: From the Avinguda del Paral·lel.
N1 (Night bus): From the Plaça de Catalunya or the Plaça d’Espanya.
From L’Hospitalet:
Route 110: From the Avinguda el Carrilet.
From El Prat de Llobregat:
Route PR4: From the RENFE railway satation.
Route 21: From the Avinguda Remolar.