Which vendors have a lot of cash?
The wiki here has a list of the vendors to sell them to that should be able to afford to give you the full value of the gold bars. Remember that they'll refill their caps every so often, but it'll involve a lot of resting.
As stated in the wiki page:
- The hologram vendors in the Sierra Madre each have 2,500 units of pre-War money. Bear in mind that you will only be able to do this before leaving the Sierra Madre as you can't return.
- The Vendortron at the Gun Runners has around 8,000 caps.
- The Quartermaster in the Brotherhood of Steel Bunker; Knight Torres often has 8,000 caps.
- Daniel Contreras the NCR Supply Merchant in Camp McCarran also usually has 8,000 caps and above.
- Arms merchant, A supply merchant at the 188 Trading Post usually has around 8,000 caps or more.
- Alexander, a Gun Runner merchant at the 188 Trading Post also has 8,000 caps or more.
- Blake will sometimes spawn with 12,000 caps or more.
- Old Lady Gibson may have 8,000 or more caps later in the game.
- Sink Central Intelligence Unit has a variable high amount, up to 24,000 caps.
- The Commissary terminals in the Divide will have around 6,000 caps, and will replenish anytime their reserves drop below 200 caps after the player exits the menu.
- Calamity in the Jacobstown lodge usually has around 4,000 caps or more later in the game.
If you have the Old World Blues DLC then:
- Speak to the Sink Central Intelligence Unit
- According to the wiki it can hold up to 14000 - 16000 caps, but I've seen it hold up to 20000 caps at one time.
If you have the Lonesome Road DLC then:
- Commissaries will hold up to 6000 caps, but once they drop below 200 caps their stock of caps INSTANTLY refreshes. So this could be an easy to well to sell the gold bars quickly (just enter, exit trading menu, reenter), but you'll end up losing quite a bit of money.
If no DLC:
- Once you have reached a high enough level Gun Runners can carry up to 8000 caps, although the time for them to replenish this stock can take a while.