Is there a Magic the Gathering inventory log?

You can download it from Gatherer and put it in Excel. I have done that for years and I could sort by cards I needed, color, rarity, set, type, etc. My Excel file is huge, so I don't hand it out any more, but I might consider selling it in the future...

I would check out the orders, the credit cards or what not that these people are sending. Also, it's not just your human error. Some people could be misspelling the name of the road they live on or even using the wrong house/apartment number. I've done that once, with some cards I bought from Ebay because I forgot to change my shipping address. This just sounds too fishy to me, and that you should immediately check with someone that can trace these kinds of things. Albeit, these are only trading cards, but they are your business and you need to secure it. The only other way is to work with the other retailers in your area or on line that deal with these same problems and try to get to the bottom of it.