It means a pile / heap of e.g. tomatoes, grain etc. So in a market the stalls might be arranged so that there are little piles of fruit or grain and you use that as the quantity. Like packets in a supermarket.
This translation of the "-na-" tense contradicts an earlier test item. here -na- is translated as present continuous ("is" doing the verb), elsewhere it is translated as indefinite ("does" the verb). For example, it is the difference between he buys and he is buying, but each time the -na- is used. Based on Simplied Swahili, -na- tense should be used for, e.g., is buying, and the -a- tense for, e.g., buys.
this is inconsistent so far and should be updated here.
I think in all spoken Swahili, you will find that the - na-tense is used for present continuous and indefinite. Usually both are accepted as translations for - na- in this course (in my experience), and where both aren't, it is probably a beta problem.
buyer is in the list of suggested meanings for mteja but it is not accepted?