Here's the official explanation...
We specifically asked the designers to make sure the Sink did not remove the items you sold to it.
The reason for this was the appliances - there's nothing more frustrating than discovering you've sold 100+ of what you thought were worthless items in the Mojave and then suddenly discover Burned Books, Cups, Toasters, etc. suddenly are valuable once the Sink personalities are on line.
It was a conscious design decision on our part, wanted to clarify.
And like the poster above me indicated, it will restock it's inventory, but the items you sell it won't ever be removed. Unfortunately, the caps balance doesn't reduce either which eventually leads to the problem where it begins ripping the player off. It was a well-intended but ill-considered design decision.
Here's how I deal with the Sink vendor to avoid problems:
1) I never use it for sell-for-profit purposes. If I'm looking to make some caps, I'll sell my loot to Gun Runners or other vendors that function normally.
2) The only things that are really worth buying from the Sink vendor are the sonic emitters, the K-9000 and LAER weapon mods, and the combination powder/primer packs. However, buying anything from the Sink means adding cap to it's caps balance, which you really don't want to do. So, if I ever buy from the Sink, I'll ensure that I sell as much back to it as I spend. Meaning if I want to buy 300 caps worth of powder/primer, I'll equal out that purchase by selling it 300 caps worth of ammo (usually BBs or Nails) in return. I sell it ammo because ammo is stackable. Better to sell it stackable items than armor and weapons in varied condition so you can avoid that "overfilled container/inventory" lag where it takes several seconds to access the thing.
The idea is to keep the Sink's balance at it's default of 14k and change. If you get it's balance lower than that, that's fine too (it'll reset back to 14k and change at some point). You just don't want it's balance to increase.
3) If I want 100% repairs, I'll use Raul, not the Sink.
This has worked well for me. I've already purchased all of the weapon mods, and the only thing I buy from it now are the powder/primer packs. It restocks regularly, and it's balance is still in that 14k and change area.