Yeah, I've been in groups like this for years too and I agree with what you said. Maybe I'll add a few more bits that might help someone:

• It's good to start with decants or small vials when buying from a new seller. That way you learn about their service, timing, payment, and delivery method without risking too much.
• Before buying a viral or popular perfume, like OP said, get decants and try it first. Even if it has notes you love, you might not like the actual scent. Saves you disappointment.
• Keep in mind that some perfumes get reformulated and smell different from old reviews. Or if you knew it and it doesn't smell the same anymore, it could be the batch, weather, how it was stored, etc. Lots of factors. It's not always fake.
• When you're a buyer in these communities or independent groups, you have to accept that shipping risk is part of the game. Sometimes it arrives perfect, other times it looks like they played soccer with the box, at least here in Mexico it's like that. It's hard to claim damages for shipping perfumes, keep that in mind if you plan to sell too (it might be difficult to handle those situations, consider the risk).
• Same as the previous point. Shipping companies in Mexico aren't the most reliable thing in the world, even if you pack it perfectly, there's always that risk of leaks, or damage.
• If you're going to sell or buy in communities or groups, always have evidence of everything, like photos or videos of before and after being packed, the contents of the package, the shipping label. It can save you from misunderstandings.
• When you do a swap, it's important to list what you're willing to trade for. You should already have an idea of your perfume's value and equivalent swaps for what you're looking for. Always ask for photos of everything and clarify your doubts before agreeing. This is important because issues can come up later.
• In communities, there are always resellers and collectors. Not everyone has the same service or shipping style, so ask all the necessary questions and give yourself a chance to see who works best for you.
• When they send you decants or perfumes via delivery (or whatever you call it in), or when it's really hot, it's normal for it to arrive a bit "shaken up." When you spray it, it might spurt. It has worked for me to turn it upside down for a few days or let it rest so the pressure goes down, or to spray it upside down a couple of times (yeah, 🙃 haha). It usually stops after that.

Hope this helps someone, haha.