Brokers are fairly straight forward.
They can be found around the cities, and in all trade skill instances. Just doublce click on the broker and bring up the interface and search for what you want. Be carefull of the 20% charge, if your buying a high priced item take a look at the persons name and location on the listing then just go to their room and buy and avoid the 20%.
There is a black market broker available. One in each city normaly out in the water (check the Harbors of course). They let you buy goods from the opposing city, but at a 40% mark up.
To sell, You need to have a room. Any room. Then you buy a market sign, these are fairly common and can often be purchased in the inn or at any furnature maker. Enter your room and place the board on your wall. Click on it, and you have a new UI to play with. You can sell as long as your online and in your room. Note you can not buy from your own room but you can browse the market.
As for other rooms and houses.. Unless you spent 10k on E-Bay dont worry about it. The prices for these are so high that only guilds, exploiters, and uber hyper players will have any reason to pick them up for months to come.
Once you have the many plat it takes to pickup and maintain these places. You can purchase or rent one, either way you still have an upkeep cost every couple of days. Inns are spaced around each city. If your with a guild or have status points you can also purchase larger houses but they still have the upkeep to drain funds.