Best to do it in the church (i found) where you can give the woman all but the item of interest to hold onto. Next go to the workbench and upgrade it and apply a mod to it to make it worth more. Have it equipped, then;
1.Hold L1
2.Hold triangle and press R1
3. 2 weapons should appear, and you just pick up and repeat
The rarer the item the more it will sell for, and currently the weapon inventory can hold 12 items (without skill upgrade). I did it with a Toxic Wicked Sickle, and made roughly 1.2k for each sold, so around 13k all up? I don't think you can do it with grenades or guns (haven't tested). Also there is another glitch for infinite diamonds, and champagne, which you can look up as well, which will help in levelling up, money and of course helping you get the coloured skull weapons