"I am buying a comfortable armchair."

Translation: Kupuję wygodny fotel.

[deactivated user]

shouldn't I say "wygodnego fotela"?

[Vengir]

No. „Kupować” is connecting with accusative form (wygodny fotel), not genitive (wygodnego fotela).

[immery]

in case you are hanging out with some Polish people- there are people who say it like you asked but that is bad Polish.

[Mim_Fox]

is 'komfortowy' not also comfortable?

[Jellei]

Not about a piece of furniture. It works in more figurative meanings. Like a 'comfortable situation'.

[Popo-lsku]

"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]

Szukać takes the genitive, whereas kupować takes the accusative. I think.

[Vengir]

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”.

[Jellei]

This comment may be a bit confusing to some people, because "kupuję" takes Accusative, so neither "fotelu" nor "fotela" are correct here.