Should drains go up or down?
Of course you know the answer to that. 
I expect most elementary school children know that drains go down.
Now, this photo is the end of a long drain line.
It begins at a newly-installed "kitchen" in a basement. From there it travels a good 20' to finally attach to this drain stack.
For most of the drain the horizontal tube is angled upward.
And where it attaches to the stack the fitting is upside down! The angle of the connection should be pointing down, of course!
The installer had a 50/50 shot at getting it right and chose poorly.
And does the white PVC plastic seal well when it is glued to the black ABS tubing? Not really. They each expand and contract at different rates, and the glue used for one does not work well with the glue for the other. These connections sometimes leak. But this comment is an aside.
So what do we have? A basement "kitchen" installed without a permit, with a sink that drains upward, uses the wrong materials, connects to the drain in the wrong direction, and glugs when the water is turned on in the sink. Why? Because air bubbles go UP!
Yes, physics works every time.
My recommendation: if your client is really excited there is a new kitchen in the basement, check to see if there was a permit pulled and closed for the work. And you might also want to bring an elementary-school child with you to see if the angle on the drain is, um, downward. Because, after all, drains do best when they go DOWN.
Jay Markanich Real Estate Inspections, LLC 
Based in Bristow, serving all of Northern Virginia.
Office (703) 330-6388 Cell (703) 585-7560
We see this often...no permits for the finished lower level...and yicks what that can mean !
LOL - I think my cat even gets this. Yes, too many homeowners do things without permits and with cheap "contractors" who are usually unlicensed and often incompetent.
Jay, I assume ( and you what I mean ) everyone understand everything plumbing rolls down hill.
That is a big issue around here S&D. And when the county finds out it can be a big can of worms.
Cheap and quick Debbie. Too bad that seems to be closer to the norm.
No Frank. Whatever do you mean? What, oh what?
Good morning Jay Markanich too often, real estate agents who list property fail to ask if permits were pulled for work done....and buyer agents should be doing their due diligence and researching in the building dept. to get copies of permits....
That sounds like a lot of work, Barbara. Isn't it easier just to blame it on the home inspector who "kills the deal?"
People, wake up... Uncle Bob is NOT a plumber!!
In this case I think it was Tio Boberto, Fred.
Billy - I think that T connection is upside down!
Tio - No comprende...
Fred - Billy Jays probably knows that's true. I said the installer had a 50/50 shot at getting it right and he, um, did not! Of course, the Tio has never seen such stuff before and just thought Elmer's glue and a fit here and there would work.