Are there private sellers or is the market dominated by real estate agencies? Is it even possible for a private person to sell an apartment or a house that he/she owns (maybe laws against this...)?

Where would you go if you need to buy an apartment or a house? Does every real estate agency have their own sites where they have their listings or are there larger sites that have listings of multiple agencies?
Do private people search for apartments online (or otherwise) or is it a job left for the buyers agent?

If some of you have purchased an apartment or a house, first hand experiences would also be appreciated.