Scammers are everywhere - Amazon, Ebay, GB, AL, even some legit looking retail websites seem a bit sus these days. I only just recently made my first purchase off Armslist a couple weeks ago - an AR lower that's been next to impossible to get directly or 3rd party retailers. Seller turned out to be great in service/communication/pricing. Only bummer was shipping cost, but that's not a knock on him, just the reality of where we live. Shipped to my FFL no problem.
If GB/AL is the only option - I usually search them up to see if sellers have a credible website/social media/reviews etc. Depending on the type/value of the item, I tend to almost always stick to online retailers with zero negative reviews. If their online presence sorta checks out, I'll reach out to them via the contact form. I then determine their credibility by the timeliness and quality of their response. Still always a risk though.
The one thing that I've been finding is that a ton of online retailers will list a product in stock, but they're really back-orders and they'll hold your money for weeks/months waiting for the item. Or they're drop-shippers that rely on distributors to ship, but aren't always current on stock levels and again end up waiting weeks/months for distributor to ship. I always email them to confirm if items are actually in stock/ready to ship or back-orders.
As always, if something looks too good to be true...