definitely a scam. don't people usually use a middleman of sorts for real-money trades?
Yes, and they never use steam cash. 99% of the time they use paypal.
Its like having a craigslist person asking for money order or some stuff and someone will pick the item up. iirc they will grab it and by the time they do the money order will bounce. Always ask for cash for physical trades, and for online trades, use paypal and have them put in the note "For virtual items; no refunds"