The metal tin used as a standard measure in Ghanaian markets, for rice, beans, maize, gari and groundnuts.
Ghanaian markets sell by measure, not by weight - by the olonka, the bowl, the heap, the bunch or the piece. There are no scales on most stalls.
Learning what a standard measure of the things you buy should cost gives you a benchmark for every market in the city. Ask a trader to show you; they will.