Because here it is about the amount of countable items.
You may use singular, when you mean:
- The amounnt of something uncountable. Attention: In Polish "uncountable" is not a strict grammatical category, so its sometimes more or less blurry. Also, some things that are uncountable in English are countable in Polish, eg. "chleb"="bread" (substance, uncountable) or "a loaf of bread" (countable); "chleby" = "loaves of bread".
- The part of ONE single object.
So, "chcę sprzedać więcej jabłka" ("sprzedawać" does not work here) would mean "I want to sell a bigger part of an apple".
I have a remark: I wrote "chcesz" because that's what I could hear. I miss to hear the ę and the ą as I could hear in Poland, specially when my friend Aldona would teach me, she had a perfect diction! She also made me understand and get familiarized with all those consonants that before seemed like white noise; but the ą and ę were so beautiful! I wonder why in this course you mostly omit those sounds.
I can't hear "sz" at the end of the word. As for the pronunciation of "ę" at the end of a word - in everyday speech it is really very often pronounced as "e" or with just weak nasality. Only the actors in a theatre say the ending "ę" in every word - in other cases it is rather considered as hypercorrection. Here is an expert's explanation on that: https://sjp.pwn.pl/poradnia/haslo/wymowa-e-i-a-na-koncu-wyrazu;7124.html