It took me less than a minute to get this achievement when I went for it, and was a darn sight easier than running around picking fights with people. Although I have to admit I got it before I became King, so I don't know if the problem others have mentioned would occur.
After buying one of the weapon's stores, simply enter it at night and do a quick recon of the weapons on offer. If all together their values come to more than 15k, then simply step outside and wait for the guard to leave (because you entered the shop one will come over) before re-entering and stealing the weapons.
3 weapons got me a bounty of 16k when the guard finally caught me, popping the achievement. Only slight problem with this is the blacksmith then hates you, increasing the value by 10%, but since you're getting 25% off for owning the place - not to mention the stupid amounts of money you get later on - it doesn't really bother me... and if does bother you, a handful of quests will get the man back on your side :)
EDIT : I have been reliably informed that this method does not work once you become King (or Queen) since the guard will no longer arrest you, so be warned.