I'm not native English speaker, but it sounds unnatural to me to put "here" in the middle there.
Which conjugation has the hird person plural ending with "i" inste of a with a tail
It's mostly because it's in the past tense.
The verb „sprzedać” belongs to conjugation class 1. https://pl.wiktionary.org/wiki/Aneks:Język_polski_-_koniugacja_I
But there are 11 or so of them. You must be a masochist to try to remember all of them by studying tables.