The disciples have freely received, now they are to freely give. Jesus sends them into the cities of Israel as the first "faith missionaries." That is, they are not to have their means of survival planned ahead of time or be concerned about how to support themselves, but only minister and trust that God will provide for them in their journey. They aren't even supposed to accumulate provisions along the way that might ensure security for even a few days. No extra cash, no bag to store stuff in, no extra clothes, sandals or staff.
The missions field presents enough difficulties without having to leap into it by total faith. Even the apostle Paul relied on the trade of tentmaking to support himself through his missionary career. But you can understand why Jesus had the disciples take this approach. For one thing, they were following in Jesus' own example, the one who had no place to lay his head. But for another, imagine if they had accepted gold, silver or copper coins, or gifts of extra clothing for doing these miraculous works of healing. Even if these were given as gifts and not payments, how would the disciples appear to be any different from a traveling circus that goes about amazing and entertaining people in exchange for money? To freely give tells the crowds that they themselves have freely received, and points people's attention to the Giver of all things.
How, then, are they to survive? Jesus tells them they may accept offers of hospitality, but even this allowance serves a dual purpose. The hospitality they find in any given city will obviously be the means through which God provides them with food and shelter, but it also becomes the gauge that tells them whether worthy souls dwell there. If the disciples are welcomed, they should stay and minister; if they're not welcomed, they should move on. They will know they are welcomed if their greeting of peace (think: benediction) is received. The remark of one commentator stuck with me here: "The peace which they wish to the household goes out and is effective; but it is not automatic, and a wrong attitude in the receiver will result in its return, like an uncashed cheque."
Jesus concludes, "And whoever does not receive you, nor heed your words, as you go out of that house or city, shake off the dust of your feet. Truly I say to you, it will be more tolerable for the land of Sodom and Gomorrah in the day of judgment than for that city." Such harsh words. Shake off the dust of their feet? Worse judgment than Sodom and Gomorrah? Just because people rejected a single opportunity to hear the gospel? Yet you have to remember that these are the cities of Israel. Since ancient times they were supposed to be preparing for this day when the Messiah would come. Their entire history has been a long journey of bearing the hope of God's promise up to this climactic point. And being the covenant people of God is a double-edged sword. While it is a privilege to be chosen and set apart, to be enlightened with knowledge and entrusted with the sacred traditions, if the Messiah should come and find that you've despised your privilege, betrayed trust and loved darkness more than light, it would have been better if you'd never been entrusted with those things to begin with. It would have been better if you'd been like Sodom and Gomorrah and rejected the way of God in ignorance. The most severe judgment is reserved for the apostate, not for the heathen.