3rd party "contractor" when buying bitcoins? i was just wondering - let's say Alice wants to buy Bitcoins from Bob - there's some kind of transfer that needs to be done (Alice sends USD to Bob, Bob transfers BTC to her wallet).

Why won't bob just disappear after receiving the USD?
If so - isn't a trusted 3rd party always needed?
You're right that bitcoin/blockchain doesn't solve fiat problems. In your scenario swap out BTC for another fiat like GBP, you have the same problem.

It's the same problem if I buy something with BTC or fiat, you have to trust the other person will send the item or have someone else offer some form of guarantee like eBay does.

So if it's high value you might enlist someone beforehand as an escrow.

Otherwise, Alice would need to sue Bob for breach of contract after Bob doesn't hold up his side of the bargain.