I ran into this some time ago after collecting a lot of junk and then selling it to the vendor at Hoover Dam. He has a lot of ammo always available, so you can unload a lot of stuff there. Anyways, if you buy more than 65536 caps (which is 2 to the 16th power, a magical processor number) worth of stuff, if you try to then sell some of your stuff to pay the bill the vendor will only credit you with the difference between what you paid and 65536. So those caps are basically "lost", because when the vendor respawns his supply it will be back to his usual total, (8000 for gunrunners, usu something less for others). I only bring this up because with the Gun Runners Arsenal add on there are lots of weps that cost 20000 caps or more, so you can get this glitch if you're not careful. Buy a little then sell a little works best, keep the totals under that 64K number and you'll be o.k. Hope this helps prevent cap loss tragedy.