Vendors will stop paying you for items that you sell to them if their caps are over 32768.
In my ignorance, I bought a few implants and WMX mods all in one transaction, which ended up putting vendor's caps a little over 33000.
Please forgive me if the method to fix this bug is already well-known, but I'll share it in case there are others that found out about this the hard way, like I did.
To fix it, you need to know the FormID of the vendor's inventory container [u]reference[/u], which in my case was Pinkerton's and took a little more work to track that down since he's not traditionally a vendor. The "Project Nevada - TTW" compatibility patch makes him a merchant, so his container is found in that mod.
I used FNVEdit to get the reference ID by clicking on the container then clicking on the "Referenced By" tab at the bottom of the right pane.
[collapsed title=FNVEdit Screenshots]
Left pane
Right pane
[/collapsed]
Using Pinkerton's container reference (4900866F where 49 is my load order index for "Project Nevada - TTW.esp") and assuming his cap count is 33000. You will need to remove at least 233 caps from it to get him under the limit (I would round up to 250 just to be safe).
Then in game, type these commands in console:
[pascal]
prid 4900866f
removeItem f 250
[/pascal]
To see the vendor's new cap count:
[pascal]
getItemCount f
[/pascal]
[size=13]Note: Using the console will prevent achievements from being earned. If that matters to you, just save the game, quit, then load that save to allow achievements to be earned again.[/size]