I live in a duplex.. there is water that smelled bad at first backing up into my basement sinks.. but now it seems to be the water that goes down my tub and kitchen sink.. and it doesn't stink.. both me and the neighbor upstairs just moved in.. the house was empty for a year or so.. when I tried to plunge it it seemed to bring out more gook.. what do I do?

Snake the main sewer line.

We need more information. Toilets flush OK? No back up from the basement floor drain? Everything else draining OK?
If everything's draining then that puts the blockage downstream from the basement sink. This can be snaked from the sinks roof vent or from a basement stack clean out if you have one. Good luck, Tom