I agree with what JB posted.
Time is money for a Domme, so I understand that they have to maximize their time to as financially successful as possible. The best way to insure their time is not wasted is to require deposits. Makes perfect sense to me.
I do not have a problem with making a deposit so long as I have confidence that they are the "real deal". It is imperative to do your homework: find reviews, read their websites and social media, even do a simple google search on the name. I think even posing a question regarding A Domme's authenticity on Domina.ms would probably get some good feedback. The deposit has to be a reasonable amount, or percentage of the total session cost and make sure you understand the Domme's cancellation policy which is almost always explained in detail on their website. Shit happens, and reputable Domme's will almost always offer some reasonable resolution in the event you have to cancel a session from which you have made a deposit.
Now having said that, I did make one exception to prepaying for a session. I had been seeing a well known Domme for a few years and we developed a pretty good professional relationship with a high degree of mutual trust. I was seeing her for 3 or 4 hour sessions so the fee was a considerable amount. Unfortunately about two weeks before the scheduled date I had to cancel a scheduled 4 hour session. This was the first time I had cancelled on her. We immediately rescheduled for the following month. She explained to me that she had been counting on the fee for some upcoming near term expenses and asked if I could send her the entire fee now and just get the session next month. I immediately said of course and I sent her the $ the very next day. I did this because I trusted her with my physical wellbeing so trusting her with a prepaid session fee was absolutely no issue for me. One month later, we had a fantastic session.
I guess the moral of the story is to session with reputable and verifiable professionals and if they require a deposit then simply cooperate with that policy. Take the time to build up some mutual trust, teat one another honestly and respectfully and everyone will win.
