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You have a drain line stoppage. A small snake ran through the drain of the sink may open the line.
Well, a blocked vent can cause generally only one fixture not to work...I know there are some pros that will blame the vent very quickly when it really isn't the problem. If all other fixtures in the house are working, I would believe the plumber knew what he was doing with the venting, and woul not think that to be a problem.
If you run water into the tub does it drain good?
I'm sorry, I mean't the shower not the tub. Does the shower drain good by itself? Put a plastic bowl over the drain and fill the shower as much as possible and drain that and see if the drain is slow or normal.
The vent could explain the gurgling from the sink into the shower but not water backing up into the shower. The air in the line has somewhere to go unless the drain line is running uphill, with a bad vent.
Yes if the capped line is the common drain, it is just filled up.