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)
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.