He is oddly obsessed with selling the T-Rex and the rocket souvenirs, probably because he has an incredible amount of them (his shop inventory shows 1000 T-Rex souvenirs for sale for 1 cap each). There are also a lot of Dinky souvenirs placed in his house next to the gift shop, as well as in the storeroom behind him.
In fact, when the player first meets him, Cliff will exclaim that the Courier has made it just in time, with only a few T-Rex souvenirs left in stock for purchase, though the truth is that he has a ridiculous amount of them. He eventually admits that "no one ever buys the T-Rexes." His shop opens at 6:15 AM.
Come Fly With Me: With a 40 Science, the player can inquire to Cliff whether his rocket toys have radioactive material in them. This dialog only appears after Chris Haversam has tasked the player with finding the rocket fuel/thrust component modules. This does not hinder dialog line to suggest rockets as igniting agent source for Chris directly.
One for My Baby: The player meets and questions Cliff about the disappearance of Boone's wife. Cliff will mention he didn't really like Carla. Like many other Novac residents, Cliff can be used to satisfy Boone's blood-lust. However, doing so will affect your Karma negatively, and you will not be able to purchase the That Gun and other unique items from him. Afterwards, if the quest is completed, Cliff will give you access to a room in the nearby motel.
Behind the locked door in his store is a stockpile of Dinky and rocket souvenirs that can be taken without penalty. The rocket souvenirs can not be resold but can be used as fuel in the quest Come Fly With Me and as gifts to some of the Mini Boomers. There are more than enough for both.
When Cliff tells you he is taking over and gives you the motel key, the dialog allows you to tell him that you were responsible for Jeannie's death and ask whether you'll still get a discount ("She missed my wake-up call"). He will act nervous and uneasy but will not take back the motel key. It also has no affect on your reputation or Karma. This does not remove the discount he gives for telling him that she sent you either.
Cliff sells the unique pistol That Gun. He will sell it to the Courier for a very reasonable price, even cheaper if you name-drop Jeannie May Crawford's name first. Alternatively, you can steal it from the room with the rockets, which is locked behind an easy locked door which can be opened with the storage room key that can be pick pocketed or bought from him.
If you have the discount with Cliff, he can be used to remove stolen tags, so you can remove the "owned" label on items you want to display in your house, with no loss of caps. Simply sell him the item, exit the dialogue, then reenter his dialogue and buy it back and the red label is gone.
Cliff seems to keep some items you sell him in his inventory indefinitely. He could possibly be used as a "pawn shop", meaning he could hold onto high priced items for a while as a loan and then you can buy them back after you have gotten more money.
Cliff's stock of Dinky the T-Rex souvenirs is limitless. Purchasing all 1000 will only remove them until his shop inventory refreshes (approximately every three days), at which point he will have another 1000 in stock.
Once the player exits Cliff's trade inventory to his dialog, all caps he has over 5000 will be removed.
pcps3xbox360 If the player trades over 30-33,000 caps to Cliff, his cap amount will not lower when the player attempts to sell items back. This makes it possible to play caravan indefinitely, as Cliff will never lose caps. [verified]
pcps3 When attempting to purchase Cliff's cowboy repeaters, caps will be deducted but when completing the transaction the Repeaters will not appear in your inventory and instead will reappear in Cliff's inventory; but as a single Repeater that has been repaired with all that he had available. Strangely, upon leaving the store, all but one of the purchased Repeaters will now appear in your inventory and the repaired Repeater that had reappeared in Cliff's inventory will still be there. A similar problem occurs if the Courier attempts to purchase a fully repaired Power Fist. If the Courier is already carrying a Power Fist, the fully repaired item will not appear in the inventory, until the Courier removes the previously used item. Upon picking the damaged Power Fist up, the fully repaired version vanishes again, unless it is equipped beforehand. [verified]
xbox360 While the step to gain radioactive fuel for the rockets is open during the quest Come Fly With Me, with a 40 Science, the player can pass infinite speech challenges while inquiring whether Cliff's rocket toys have radioactive material in them. Note this speech challenge gives no XP, and the speech challenge dialog does not appear at all unless the player has at least a 40 Science (as opposed to normal speech challenges which appear as dialog options even if the player will fail them). This can easily be exploited to rack up 50 speech successes very quickly for the Outstanding Orator achievement. This dialog choice will disappear once the player has obtained the radioactive fuel, by any means. [verified]
pcps3 If the player barters with Cliff for the souvenir rockets during Come Fly With Me and then later completes The Screams of Brahmin the barter dialogue for the souvenir rockets will reappear. If the player waits in the shop after this occurs, Cliff will disappear from the game. [verified]
xbox360 Since Cliff keeps nearly everything that the player sells him, if the player sells to him extensively throughout the game and his inventory becomes very large, interacting with his inventory items will become increasingly slower the more the player sells him. [verified]
xbox360 Upon the odd occasion (seemingly random), once the player has bartered/played Caravan with Cliff and tries to walk out of the Gift Shop, the game will black out (where the loading screen is supposed to be) and essentially crash, causing the autosave file to become corrupted and unplayable. [verified]
pc When asking to buy from Cliff, the game may crash. [verified]