See these threads for information about the vendor cap bug with the Sink CIU vendor in Old World Blues:

http://www.gamesas.com/index.php?/topic/1214975-the-sciu-vendor-bug-yes-devs-its-a-bug
http://www.gamesas.com/index.php?/topic/1213924-sciu-trade-lock-out

A few of the other threads I've seen on other forums (and the mods on the nexus) that attempt to fix this problem are trying to reset the caps via the vendor chests or the CIU computer object. As mentioned in the first thread above, the caps causing the trouble are actually being held on to by the invisible NPC vendor.

The object in the GECK we are looking for (assuming your load order is FalloutNV.esm > DeadMoney.esm > HonestHearts.esm > OldWorldBlues.esm ) is:

0300F06C (NVDLC03SinkMainComputerTalkerREF)

Since it is invisible, you will need to do the following to activate it. Fire up the console (by pressing "~") and run:

prid 0300F06C

You can then verify that it is indeed holding on to the caps, and see how many caps it currently has in it's inventory by doing:

getItemCount 0000000F

To remove a number of caps (let's use 20,000), do the following:

removeItem 0000000F 20000

You can then run getItemCount again to verify that the caps were removed, and repeat as needed. Once you get it back down to a reasonable number (under 32,768 for some / 65,536 for others) you can resume dealing with the vendor, and just be sure to keep an eye on how many caps the vendor has going forward.

If you are like me and realized you sold off a bunch of great gear and got zero caps in return for it, you can also find the chest that holds items sold by the player. This object in the GECK is:

0301543B (NVDLC03SinkVendorContainerREF)

You can warp to that chest by doing:

player.moveto 0301543B

Once you pull out any of the items you didn't get fairly compensated for, you can just step off the chest and fall, and you will land back in front of the CIU vendor.

Hope that helps someone!