Three of mine are from Kijiji! Meet in public with someone you know. Good point about scams though. I had someone want me to send them money & they’d send the budgies in a cab. Yeah, along with a dodo, I’m sure. This was at the height of budgie rarity here.
I met Oscar through Kijiji, but I agree that scams are everywhere unfortunately.
I have a Kijiji alert for "macaw" in Ontario and I find that seeing the steady stream of ads not only provides a sense of what a legitimate post looks like, but also tunes your radar for ones that have warning signs (after a few years of doing so, it feels like that's the case).
My best advice is don't buy sight unseen, but a close second might be to avoid getting too excited too soon about a specific bird (clouds rational judgement) and be prepared to walk away. Perhaps related to that advice is to have an impartial friend review an ad you're interested in and also use them as a sounding board for what you're hearing from the seller if/when you open a dialogue with one (or use this forum if your friend isn't knowledgeable about birds).