I know someone who has done pro-sub work without, as far as I know, any more problems than usual.

Personally, I'd want someone I trust within hearing range, but this is not always going to be possible. The client is going to have to know you have a silent alarm in place, and I'd want photo ID or similar, something that says "You will be traced if you misbehave."

There is, of course, bondage that looks nice but is easy to escape from. Having "one pull to untie" half-bows with reach, for example.

I've worked with professional submissives and I never saw a problem. But that's because they worked in  a very safe enviroment. You need to know the rule of the thumb: You will never be alone with the client in an apartment.

This is quite a lot of trouble, especially if I'd like to do outcalls in future I guess every possible precaution is needed for safety though.

You need someone on the next room. Not only that but she needs to check you out (maybe take a little peek thru the door or something every 15 minutes). If you can't follow these guidelines I strongly reccomend NOT to become a professional sub.

I'd think having someone else around would put off a lot of clients.
I'll have to think about it, it may be more trouble than it's worth. I guess this is why there are so many pro dommes, but so few pro subs.

There's definitely a market for pro-subs + personal services as I have found out!  I do it - only 1 session a day though.  Seriously, if you can get it right, there's plenty of work - currently I'm booked up for the next 7 weeks, yes, 7!!  It's madness..but nearly a year in, the majority are return clients and I only see 1 maybe 2 new clients a month (who then turn into regulars, hence the long wait to a free appointment).

What I have found is that a very comprehensive profile, with clear boundaries, etc, works very well.

I also have a good friend of mine (male) who remains discreetly on the property during sessions and this is on my ad as well.

I ask them to email requirements and only once I agree to meet them, do I invite them to put a booking through. 

I have strict processes for vetting and do not veer from them - I've never had a problem, but yes, it is riskier, but everyone has been very respectful and friendly.  Of course, I get the odd timewaster email  (I do not advertise my telephone number), but woman's intuition/gut instinct/uncomfortable with what they say/how they write, etc and they get a polite "thanks, but no thanks".

Every session is different - every dynamic created in the Dom-sub arena is different, so 2 sessions are never alike.  It's great fun!