I mainly use Backpack.tf to find shit I'm looking for, so you could use that, and you could also use Steam Community Market, but stuff's overpriced there so I recommend looking in backpack.tf first.
And there's also Mannco.store, which is very similar to marketplace.tf and is probably gonna be the new Marketplace.tf soon.
As for avoiding getting scammed, beware of impersonators (Make sure it's their actual profile, check inventory, steam level, etc.), never accept private profiles or people with nothing but achievement items in their inventory, always a scam. If someone asks you to join their team claiming they'll pay you, it's a scam. Check trade offers very carefully. And if something seems too good to be true, it probably is.
If you ever feel like something is suspicious, you can ask someone who you think is experienced, you can even ask me at anytime man, I wouldn't mind helping you.