I seem to recall one or two doms would ask something similar from prospective subs.
I have no idea why it woukld make the slightest difference,
"So you would like to book a session for two hours. that will be £500. But unfortunately you are a supermaeket trolley collector so I can't see you"
Oh yeh I can see that happening. 
For a Dom asking a prospective sub it will be the same reason as a Sugarbabe asking a Daddy, basically work out what his job is so they can work out how much he earns and is likely to have saved / invested etc so they know how much they can rinse the guy

Anyway best to tell them you aren't working as you've only just been released from prison, when they ask what you were in for you could tell them any old shit like killed a prostitute but the reaction would be

Obviously that's a joke, don't tell them that as it would seriously disturb someone.
I normally just make some shit up like an old job from the past or something that only a couple of people in the country do -
Water diviner; Fletcher; Town cryer; work in a factory I put the jam in jammie dodgers which could actually be true in Blackpool as Burton biscuits are here, just thought of a new one I make Blackpool rock