I just had points transfered into my account this week.
I offered to pay via paypal before the transfer went through, but the other member preferred for me to pay with a check. I offered to send the check first, but she said she wanted to do the transfer first and then have me send the check.
So, she did the transfer, and I sent the check.
I've also transferred points into another member's account and she offered to pay in full upfront. Once the payment went through via Paypal, I called MS and transferred the points.
Two transfer transactions, Two great outcomes
Trust is a huge factor when it comes to both transferring and renting. I've rented points to 7 different people (we're going on a cruise

) and I've done two transfers. You HAVE to trust the person you're working with! And make sure you lay your terms out clearly and early.
You need to do what feels comfortable for you. If you only feel comfortable with getting the payment first, then only transfer to someone who is willing to pay first.