Similar experience. Ordered 2 Mosins online, showed available, charged my card. Got a voice mail a week or so later "Sorry, ran out. We'll process a refund." Hopped on the phone with another vendor and had my rifles in about a week.
It happens, but they [you] need to better match actual inventory with the web site. Nowadays this isn't rocket science, and when dealing in limited inventory should be Job 1. Anything else leads to upset customers that will go elsewhere next time. Much easier to not take an order you can't fill anyway.