New sellers beware of all the dirty tricks and manipulation that goes on. I had 10-13 fake orders created all about the same time in my FBA account tying up inventory and saying "pending, awaiting payment" all of them. It's obviously a scam. It's very upsetting that people are so dirty. I have reported to seller support.
You are correct, that is how the site works. The buyer does not use a fake credit card.
It is your competitor who places your product in their cart, which subtracts it from your inventory. If they never press the purchase tab, it can sit there for weeks before dropping off. Remember, when it does drop off over time, the product has been deducted from your inventory and YOU are responsible for placing it back in inventory.