I live on the third (and top floor) of a condo.
When I moved in there was a strong sulfur smell coming from the bathroom sink.
The entire bathroom has been renovated including replacing the trap under the sink.
The smell is still there. I have put everything imaginable down the drain - liquid plumber, baking soda and vinegar etc to no avail.
It seems only to smell while the cold water is running.
I had a plumber tell me it is the DWV (he took a look from the roof) and that they had to be cleaned.
If that were the case, wouldnt there also be a smell when I run the hot water?

You may be looking in the wrong place. Your lavatory has lots of space inside for nasty bacteria to grow. (see image) try this, remove the trap and block off the open pipe with something. Now, using a funnel, pour 1/2 gallons of bleach down the overflow. Now fill the bowl up until it runs into the overflow and you can't fill any more. Let it set for an hour or so and then reassemble. Does is no better now? Let me know if it works for you. Good luck, Tom