There are 2 vendors that carry a lot of caps, gun runners and Sgt. Contreras at McCarren, they both carry around 8000 caps.
If they don't sell enough stuff you want to buy to equalize the purchase, buy expensive ammo types that you don't use:
For example I don't use .308 / 50. mag bullets. So buy that from them to equalize the sale of the gold bar (this is assuming you have a 100 barter skill or your wearing outfits/using chems to have 100 barter skill)
Use the .308 or 50. mag ammo as commerce for other vendors who have considerablly less caps to offer. Use 12.7mm or whatever ammo you don't use.
There, you have gained every cap from your bars.
If you are enemies of the NCR like I am, just use the NCR engineering suit as a disguise to get into McCarren.....or just shoot your way through, Contreras doesn't care what side you are on.
