Valid reasoning, but I still can't really believe that you can just live off of the income generated by a shop only accepting BTC. Even if you ran that shop without paying taxes, etc. it wouldn't be enough, most likely.
That is why it would have to be a side business or hobby. Additionally, many people living in more rural areas or that are self sufficient don't need much of an income and have their own gardens and trade and barter locally. Some of the businesses in that directory likely do that as well. I.E.. Technically I can live with a salary less than 2k a year If I wanted to as I am almost completely self sufficient on my permaculture farm. Fiat income does not necessarily equal quality of life either... some of us live in paradise for almost nothing.
...but how are you able to prove you're not spending the coins at some point over the next 5 years!?
Many ways to do this....
1) deposit btc in publicly audible blockchain address where clients can see their purchases deposit in realtime and audit the business is keeping their word. Reputation would be maintained and easily verified by anyone auditing at any time. All outputs would be seen in realtime and ruin the businesses reputation.
2) payments made to multisig address where a majority of keys held by outside escrow agents to insure that funds aren't spent beforehand
3) Payments transferred immediately using nLockTime or soon to be incorporated checklocktimeverify to lock the btc on the blockchain and make them impossible to spend within a certain amount of time. This could be verified and audited in a decentralized matter like option 1.