If I remeber correctly, as long as two basic things stay the same, gun mods are legal. Those two things are function and serial number. If you have a semi auto pistol, serial # a123, as long as the gun retains the serial #a123, and has not been made into a class 3 weapon(like sawing down a shotgun to under 18"barrel length, or converting a AR15 to full auto, etc.). It is legal to buy and mod as much as you want. Just remember that while you can make a handgun into a long gun with no problem, DON'T turn a long gun into a handgun(under 26inches in length), as this is illegal under federal law. The only possible problem is if you sell too many guns, they may think you are involved in something illegal should they become aware of it. So i would say, if you are buying, then moding, then selling, more than five or six guns a year, you may want to get an ffl and keep records, as they would probably consider this a business venture. Oh, and always remeber that state and sometimes local laws may place additional restrictions on top of the federal and PA laws that I am familiar with. So it is a good idea to check the laws anyway.
