Ok, you are in a pickle, selling knives usually gets you scammed, but here are a couple tricks to avoid that.
1. Use a trusted mpgh middleman, and make sure it isn't a fake, then paypal should be fine.
2. Use BTC, but no one's gonna want to go first, because you can litteraly grab the money and take off
3. Trade it down to things worth less than 400$, or a bit less preferably, and sell them on steam market (you only get steam money tho)