"I am buying a comfortable armchair."
Translation:Kupuję wygodny fotel.
shouldn't I say "wygodnego fotela"?
No. „Kupować” is connecting with accusative form (wygodny fotel), not genitive (wygodnego fotela).
in case you are hanging out with some Polish people- there are people who say it like you asked but that is bad Polish.
Not about a piece of furniture. It works in more figurative meanings. Like a 'comfortable situation'.
"Fotel" and "pokój" are both maskuline!? "Szukamy wygodNEGO pokojU" but "Kupuję wygodNY fotel". I do not understand the different endings. What about: "Kupuję wygodnego fotelu" ?
Mim_Fox is right.
Also, although the form „fotelu” is correct in genitive, at least according to Wiktionary, the form I personally see more often is „fotela”.