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.
"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" ?
Szukać takes the genitive, whereas kupować takes the accusative. I think.
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”.
This comment may be a bit confusing to some people, because "kupuję" takes Accusative, so neither "fotelu" nor "fotela" are correct here.
Notice that there were 2 sentences and the reply was referring to the sentence in the genitive form.
Yes, but the sentence that used Genitive didn't mention an armchair, and the sentence that mentioned an armchair used Accusative.
is 'komfortowy' not also comfortable?
Not about a piece of furniture. It works in more figurative meanings. Like a 'comfortable situation'.