You need to order from site sponsors or other reputable companies. If I order from a company like Rainier, Right to Bear, Primary Arms, etc., I trust their inventory and have never had a problem. And that's after placing literally hundreds of orders through these dealers. That's no joke, either. If they let me put it in my cart, I've always received it. Maybe I'm lucky, but I believe the guys that keep live inventories on their sites are usually very accurate. Some are so accurate that their site IS their method of keeping inventory.
Here's an inside tip. If you're placing orders on "local gun shop websites", you're bound to deal with a large wait or just other problems in general. Most of them don't have the inventory on hand, they simply have the item drop shipped to you via their distributor or even worse, they order it and once they receive it, then it goes out to you. As an employee of a local custom gunsmithing and sales shop, I appreciate you're effort in supporting small local businesses, but if you wanna buy from them the best practice is to GO THERE and talk to them. Tell them what you're looking for and see if they have it or if it's available to them. If it is, ask them to be very clear on the wait time to receive your item(s), especially if they expect you to pay up front or provide a deposit on said parts.