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.
is 'komfortowy' not also comfortable?
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" ?
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.