any websites
Put in google 'zamiana mieszkań kraków' and check the results (in Polish)
Overall the most popular classified ads sites in Poland (not only real estate) include:
gumtree.pl,
olx.pl,
gratka.pl,
otodom.pl (this is typical real estate).
And other such pages which may be also useful for you:
nieruchomosci-online.pl
domiporta.pl
nieruchomosci.net.pl
oferty.net
Some basic words translation into Polish:
real estate - nieruchomości
swap - zamiana
apartment - mieszkanie
house - dom
agency - agencja
But any regular Polish-English translator should do the job quite good.