I have never been scammed luckily!! And I do a lot of trades across various different websites!
I've only had two incidences that rubbed me the wrong way. The first time, I went to an island where a villager was crafting. When I arrived, the host already had a nearly full island (I usually avoid islands that are full due to the incredible amount of loading screens and disconnections).
I dropped a tip of like 50k, they said thanks and picked it up, and I started heading towards the villager's house. People started leaving the island and with every step I took there was a loading screen LOL.
I eventually got inside the house but the villager had stopped crafting

The island host was like "Oh sorry everybody!" and ended the session without giving the tip back. It wasn't a big deal I guess, maybe they just got overwhelmed and forgot? But nevertheless, nowadays I usually give the tip afterwards. xD
The second time, I was letting people from Facebook visit my island for a DIY. I was asking 1 nook ticket per entry. 1 person arrived, I said hello but got no response from them. They ran straight to the villager's house, and back to the airport. No goodbye or thank you. They also didn't drop the ticket, so I messaged them on Facebook asking where they left it because I couldn't find it. They didn't apologize but said "oh I forgot, I was in a hurry. I'll come back later" but I had a trade I had to get to so I closed my gates.
A little while later they messaged me saying "your gates are closed" but I was already frustrated by their attitude (not bothering to say hi, thank you, or even a simple sorry for forgetting the ticket) so I just said "yeah because I had somewhere to be as well." and they didn't respond lol. I wouldn't consider this a scam because maybe they did genuinely forget, but they didn't seem apologetic at all about it. If they had said sorry I would've said its fine!!