As the topic titles states, I cannot buy anything from anyone because the game crashes whenever I attempt to do so. Here is my load order if it helps.

FalloutNV.esm
Fallout3.esm
DeadMoney.esm
HonestHearts.esm
OldWorldBlues.esm
LonesomeRoad.esm
GunRunnersArsenal.esm
Anchorage.esm
BrokenSteel.esm
PointLookout.esm
Zeta.esm
AmmoSpreadEffectFixer.esm
Companion Core.esm
FNVToolkit.esm
Advanced Recon Tech.esm
Caliber.esm
Decrucifixion.esm
Kaya47NF.esm
More Perks.esm
More Perks for Companions.esm
More Perks for Honest Hearts.esm
More Perks for Old World Blues.esm
NVR-Strip.esm
RobCo Certified.esm
SpeedyResources.esm
War Never Changes.esm
WMVM.esm
iHUD.esm
D.E.I.M.O.S..esm
RFCW_PackageFix.esm
RFCW_PackageFix_Anchorage.esm
RFCW_PackageFix_BrokenSteel.esm
RFCW_PackageFix_PointLookout.esm
RFCW_PackageFix_Zeta.esm
CASM.esp
LMDExpandedHotkeys.esp
SweetVeronica.esp
Brotherhood and House Alliance.esp
YangtzeBunkerPlayerHomeV1_5.esp
PerkEveryLevel.esp
XFO - 4bb - Perks - Two Perks Per Level.esp
XFO - 6aa - Epic Skills - Effects Over 100 (NVSE req).esp
Light My Flare.esp
ExtendedNVRadio.esp
NewVegasBounties.esp
NewVegasBountiesII.esp
War Never Changes.esp
More Perks Update.esp
More Perks for Companions Update.esp
More Perks for Honest Hearts Update.esp
More Perks for Old World Blues Update.esp
NewAndImprovedPerks.esp
Decrucifixion.esp
FalloutNVCheatTerminal.esp
WMVM-OWB.esp
Homemade Grenades.esp
Advanced Recon Gear.esp
Advanced Recon Tech.esp
Advanced Recon Gear-Locations Doc Mitchell.esp
Ahzteks Weapon Overhaul.esp
Camon MG42 and SS Outfit.esp
Murdelizer.esp
Omegared99-M2081.esp
Survivalist Crossbow.esp
turrets.esp
Colossus NV.esp
RobCo Certified Friendly Hit Fixer.esp
Level Cap 100.esp
GrenadeHotkey.esp
WeaponModsixpanded.esp
WMX-ModernWeapons.esp
WMVM-WMX.esp
WMVM-WMX-MW.esp
Nevada Skies.esp
ScorpionAndSpiderRifles.esp
CIAO-Complete_Incendiary_Ammo_Overhaul.esp
Boom Boom.esp
BerettaNV_NVSE.esp
FuturisticAssaultRifle.esp
Nordic Customs.esp
RFCW_Main_LonesomeRoad.esp
RFCW_Main_HonestHearts.esp
RFCW_Main_DeadMoney.esp
RFCW_Main_GunRunnersArsenal.esp
RFCW_Main_OldWorldBlues.esp
RFCW_Main.esp
RFCW_Anchorage_Main.esp
RFCW_BrokenSteel_Main.esp
RFCW_PointLookout_Main.esp
RFCW_Zeta_Main.esp
RFCW_MagicSewerTravelPoint.esp

Total active plugins: 91
Total plugins: 126

Its probably a mod. What happens when you disable all mods (uncheck them) and disable archive invalidated?
I also recommend you backup the fallout.ini and falloutprefs.ini and let the game remake them. They are the saves folder. You can restore them later.