I have heard that a m'zuza must be checked twice every seven years by a qualified sofer. I have also heard that, assuming it's known to have been kosher at one point, it need only be checked for faded or cracked letters thereafter, and that that check can be done even by a layman who knows what the letters look like (provided he take any questionable m'zuza to a sofer for expert checking). Which is correct? Sources please.
I am a certified sofer. I will tell you my opinion. First I would like to ask you to take a look at my website, particularly at my blog, Diary of a Sofer. I have digital pictures of many shailos that come up. Most days I see shailos, either in mezuzos, tefillin, or where mezuzos are put. Many of the shailos are in mezuzos that have been checked. Sometimes people have missed some pretty incredible things, as you will see if you follow my blog. If you have high quality mezuzos, and those mezuzos have been computer scanned, and they have been checked by a certified sofer, and you don't have any special problems with the writing, it is possible that you may do a fine job. There are two problems however, that I see. One, you may not know keses hasofer, and mishna berura on these inyonim. There are some central halachos that you need to know before you work with mezuzos, or you can end up posseling something, and never knowing it. If you are never planing on touching a knife or a pen, you certainly have less chance of messing up, but I couldn't recommend it unless you were familiar with the main halachos of stam. I don't mean you have to take a test on the halachos of how to make dio, but you should know what chok tochoks, sho ksidran, kiddush Hashem, mukaf gvil, and leshma are. I just can't believe that without knowing the basics of those halachos that mistakes won't be made. My website is mezuzadepot.com and you will see many shailos there.