So a purchase may be a bad idea. In such cases, you should consider making your own shotgun. One process that requires no particular skills, save the ability to use glue without permanently embedding yourself in your project and a drill without inflicting lethal perforations, is as follows:
Paper strip, approx. 2" wide, several feet long (or multiple strips)
Paper strip, approx. 3-4" wide, 2-3' long: Alternate - wooden dowel, diameter equal to shotshell
Elastic band (rubber band or inner tube rubber)
Nail (16P is fine)
3 to 5 magazines (yes, the kind you read)
Glue, tape, twine
1. Obtain a shot shell of charge and load suitable to your application. (This device will be rather less than a full length long arm, but a bit more than a pistol; a 12 gauge may be more than you wish to deal with here, although I have seen it done.) Start by taking a long strip of paper, and gluing one end to the length of the shot shell. The edge of the paper should be flush with the rim at the base of the shell. Now you should roll the paper tightly around the shell. The intent is to increase the diameter of the shell case to equal the rim diameter.
Whether or not the paper also extends beyond the crimp of the shell is immaterial. When you have finished rolling, glue the paper end in place so that it will not unroll.
2. For the second part of your tool-of-defense-to-be, you have some options. One is to take a 16 penny nail and wrap paper around it (as with the shell above), but leaving the nail loose within the roll so that it can freely slide through the center of the roll. The roll should equal the diameter of the rolled shell assembly, and be a quarter to a half inch shorter than your nail. Before making the final few revolutions around the nail, stretch a strong rubber band (or possibly a length of rubber from a tire inner tube) across one end of the thick-walled tube which you are forming. Glue each end in place, and continue with the last few wraps of paper.
Alternatively, instead of forming a tube of paper, you may wish to use a wooden dowel with a diameter matching the shot shell assembly. In this case, you will need to drill a hole the entire length of the dowel. The hole should be just wide enough to allow the nail to slide freely. Again, you will need to stretch an elastic band across the hole at one end.
The nail is going to be the firing pin for our improvised shotgun. While not absolutely required, it is best to file the point down until it is slightly rounded rather than sharp. Slide the nail into your tube/dowel so that the point extends beyond the end of the tube and the nail head is covered by the elastic band. This is your firing assembly.
3. Now place the firing assembly end to end with the shot shell assembly so that the nail point contacts the shot shell primer. Gently press the firing assembly flush against the shot shell base and tape the two sections together. Note that at this point you can now fire the shot shell by pulling the nail back against the tension of the elastic band and releasing it. The safety challenged among us should also note that this device has no safety; be careful.
4. Next, take the magazines and sanitize them. By this, I mean to be sure you haven't left an address label with your name and address on them. It would be a terrible shame to go to all this trouble to construct an untraceable weapon only to leave the police your calling card.
Once that is done, roll a magazine very tightly around the shell and firing assembly. The firing assembly should be flush with the edge of the magazine, with the firing pin/nail extending beyond it. By now, this rolling process should quite familiar to you. Tape or glue the magazine in place so that it will not unroll. Repeat this process with at least two more magazines. Do not roll so many magazines that you cannot get a firm grip on your new weapon. After the final magazine, wrap the roll with tape or twine.
You have constructed a shotgun. Compared to a steel-barreled gun, this one is very short range; say, mugger-range. (And won't that mugger be surprised when the supposedly helpless bookworm takes him out with an armful of reading material!) As stated before, it is also a single-use device; once fired, you merely dispose of the incriminating evidence by tossing it into a convenient dumpster, or even by incinerating it.
When firing, particularly with a larger gauge shell, be prepared for significant recoil. Grip the weapon firmly. Bracing it against a hip may well be advised. Point it at the offending aggressor, pull back your firing pin, and release. In addition to the recoil and muzzle blast to which you may already be accustomed, you can also expect a shower of confetti. Think of it as a celebration of the elimination of a goblin. But also remember to carefully brush any off of yourself.
Perhaps it has occurred to you to wonder if this process can be applied to other cartridges than only shot shells. It most certainly can. I believe that you might find such a gadget scaled down to .45ACP (or even .38 Special) to be quite manageable and concealable. With a little imagination, you can probably think of several occasions when a disposable zip gun which the usual metal detector will overlook could be handy in the extreme. Trips through federal buildings and airports spring to mind. Be creative.