Lex,
I'm sorry to hear that other people have experienced this too.
The whole point of shopping online is to avoid these issues. I mean, I've basically resorted to calling every time I order online and checking on the availability. Unless a website specifically states that they are in stock (or even better, give you a quantity remaining), I will only trust someone on the other end.
Here's a related pet peeve that used to happen before I started calling: You order several things, some of them are in stock and one or two aren't. Instead of shipping the ones in stock and waiting to send the rest, they stop your whole shipment. I actually ordered a grip, stock, and gas tube from a well-known company. I called a week or two later after not receiving a tracking number. They were like, "Oh, we're out of gas tubes, we should get some in a couple/few weeks. We'll ship everything then." I was like, "Well, since I ordered about $180 worth of other merchandise, do you think I could see that sometime before next month?" (I said it much nicer). She agreed to ship the stuff that was in stock. I firmly believe that if they have to pay two shipping costs (one at their own expense), it may motivate them to upgrade their online shopping cart technology.
However, I ended up buying a gas tube from a local shop because I didn't want to wait to put it on my XCR. I just called and cancelled the tube. They were very nice, but that was not the issue.
So now, except for a few specific websites that I know keep accurate inventories, I save myself the trouble and call ahead. Then I usually end up placing the order online anyway.
Ahhhh! Feels good to rant every once in a while.