Stinky water in bathroom sink

If you are running the bathroom fan, it may be sucking air back up through the Ptrap when the water runs. Or, you may have a bad air admittance valve under the sink. You can easily change the latter yourself for much less than a dealer will charge you to do it.

If neither one of those seems to be the issue, try thoroughly sanitizing your entire water system. There may be something growing in a section of water supply line near that sink, that got missed during sanitizing.