most reputable SP's require deposits for new clients. Personally I must have paid 4 or 5 deposits in the last 45 days. If the lady is well reviewed on here its pretty much a safe bet that she is providing actual services and not a scam.
Deposits for new clientele is done for a few reasons, to deal with no shows and also as a deterrent for competing providers to make fake bookings, also its part of the safety screening process as when you send an EMT it tells the provider the legal name of the account holder trying to book.
Personally speaking ive never had a provider cash in a deposit I have sent via EMT via a corporate account. I just send it to confirm I am serious and I pay them full amount of cash at the session and cancel the deposit afterwards. Most SP's dont want to accept the deposit either, since when they cash it in, it tells the sender of the funds the legal name of the person of the account which received the money which is a nono for anonymity.