Hi
Good questions.
Once the number of resource credits you have falls below the cost of the transaction you want to make you can’t make that transaction.
For example, if it costs 1.4 billon resources credits to comment on a post and your balance is 1 billion, you can’t comment. But a cheaper transaction like a Steem transfer costing 500 million resource credits could be done. So it’s a “pay to play” system.
Your second question is actually also one of the most frequent questions and the answer is no.
Steem developers didn’t want to create another commodity that people would sell, so they made resource credits nontransfersble for now. They do regenerate daily and if you buy more Steem and power it up to Steem Power you get more resource credits credited to your account.
Great questions!