Fallout: New Vegas

You can also get this if you gather 10,000 caps worth of stuff TOTAL in your inventory. Then go to the Gun Runners (near Freeside), and save the game. Sell EVERYTHING in your inventory to the vendor, and once you get the achievement, reload the save.

*Note* I believe that you are initially only capable of selling 8000 caps worth of stuff to the vendor, since he only has 8000 caps. So, all you have to do is buy back some of your sold items (or his items, it doesn't really matter), and sell them once again to the vendor until you see the achievement pop up. Thanks to Vinicius Menti for pointing that out.

I recommend you use the old Fallout 3 Repairing Trick. Boost your repair skill early (which is always wise) and repair and sell weapons for high gains. The Jury Rigging perk can help repair expensive weapons with inexpensive weapons, maxing cap gains.

At the beginning of a game Tag the Repair skill and as you level pour skill points into Repair to get it to 90 ASAP. I suggest raising Intelligence to 7 at Character Creation and racing to New Vegas at the beginning (avoid sidequests, do main questline) to get to the New Vegas Medical Centre and purchase another Intelligence point implant at low level (giving 1 extra skill point at levels). Be sure to collect snowglobes on the way, the ones at Goodsprings, Lucky 38 Casino, Vault 21 and Old Mormon Fort are easy to get, and give them to Jane in Lucky 38 for 2000 caps each (these do not contribute to the achievement). The Comprehension perk can also help to temporarily add 20 points of skill for Repair and Bartering with their respective magazines (and many other things too). Try to get the Bartering skill to at least 50 as you level also.

Collect the following weapons on your travels and keep them in a safehouse until you reach level 14:

.44 Magnum Revolver (2500)
10mm Pistol (750)
Hunting Revolver (3500)
9mm SMG (850)
10mm SMG (2370)
Sawed Off Shotgun (1950)
Laser RCW (2150)
Laser Rifle (800)
Plasma Rifle (1300)
Incinerator (1300)
Rebar Club (500)
Combat Knife (500)
Super Sledge (3000+)

The prices indicated in brackets are when these weapons are near 100% condition. These weapons are not too rare and many can be found on Humanoid enemies like Powder Gangers, Super Mutants and the various "Raider" gangs. Vault 3 can be a very good place to horde some of these weapons. They are also middle range weapons (there are better versions) so you shouldn't miss them if you sell them. Also collect every other weapon you can find to use with Jury Rigging.

Whenever you collect one of the above weapons NEVER use one to repair itself (i.e. don't use a 10mm Pistol to repair another 10mm Pistol). When you reach level 14 take the Jury Rigging perk (need Repair 90). At this point you can use other weaker and less expensive weapons to repair these weapons you have collected (e.g. 9mm Pistol can repair a 10mm Pistol/9mm SMG/10mm SMG or the .44 Magnum). This will max the caps gains you can make when you sell.

You can maximise a sale by wearing the Roving Trader Hat and Clothing (+5 to Barter with each = +10 Barter overall) and reading a Salesman Weekly magazine beforehand (+20 w/Comprehension). Always sell in bulk to conserve magazines.

If you want to keep one of the above weapons for yourself do so (I usually collect Unique weapons for use). Don't bother to apply mods to things you will sell.

I use Victors Shack in Goodsprings to stash gear as it has everything you need inside and nearby too. Up to level 14 sell Aid and Misc. items you know you won't use and that ideally has a value/weight ratio of 10+ caps.

You can also apply this method to armour, but the gains aren't as large and armour does tend to wear out on a PC quickly. It is better to save armour for repairing armour you use with Jury Rigging throughout the game.

This achievement really doesn't require any special trick. In my first run-through i had this achievement by the time i got to i-118 and by the time i was ready to pick a side in the war i had 200,000+ caps. Just get a companion early on to help with scavaging/carrying crap. Boone and ED-E are pretty easy to attain early each comes with roughly 210 lbs worth of storage. pick up even junk like tin cans. If you find better stuff later and before you can sell it just drop the cheap junk and pick up more expensive weapons, etc. It might seem like a waste to sell garbage at .7 per unit but it quickly adds up and if your not at capacity your just wasting free storage room.

while your exploring the wasteland, if you dont care about your karma or whatever, get some really good guns e.g (alien blaster) and then sell it to a travelling merchant, once you have sold it, kill them to take your gun and a lot of caps back.

If you care about your karma, the best way is just to go into casino's. You should get it while doing the casino achivement (get banned from all the strip's casino's)

If you have a compainion while walking around the wasteland find really valuable stuff, for example, guns, sensor modules, and various other stuff. Give these to your campianion so that when theres a vendor,merchant, just take the stuff off your companion and then sell it

Honestly If you do the brotherhood missions find all the T-51b armor and sell it plus junky weapons in my opinion nearly all guns are since i use OP weapons but anyways save if you want to sell the stuff to gun runners achievement unlocked in no time doesn't require some huge guide to figure that out.

Good luck