It would be foolish to just blindly accept your offer. Nobody will go such trouble just to prove it to you that he can do it without any prize. And I think your question can easily be solved by contacting someone you know in real life and making the guy buy things that you need. In return, you will pay him in bitcoins.

No matter how you put it, bitcoin is still a currency and it can easily be used to buy something in real life even without using fiat. People will always accept your offer (like buying groceries for you) as long as you pay them. So using only bitcoins to survive is possible.