I find that it's only an issue for the smaller sales. If someone wants a domain to the tune of paying thousands $$ then they're serious enough to not be scared/annoyed off by having to spend 5 minutes setting up an account with a new registrar. I've never lost a larger sale by asking the buyer to set up that account.
However, with smaller sales it's been hit-or-miss. If they're buying a domain for xx or xxx, then if they're only 'lukewarm' on the domain, sometimes they just feel it's not worth the bother to open an account with a new registrar. Again, if they love the domain (and they don't feel they are being scammed), it's been no problem opening the new account.
I just lost a sale over the weekend. Did outbound Friday for a domain, priced at $650, buyer said they were interested and let's proceed. When I mentioned they needed to open an account with the current registrar, I didn't hear back. I followed up Monday, they responded that they're no longer interested. It's a case where, if the domain had been ready to transfer instantly, I expect I'd have transferred it right away and I'd have a sale.
The new-registrar-account thing sometimes kills a sale outright; sometimes it just makes them put off the purchase for awhile, and during that put-off period they change their minds (which is what happened with this non-sale I just mentioned); and sometimes it doesn't deter them at all because they're hot for the domain no matter what.
To sum up:
1. - I find it's never been an issue, when it's a larger sale (x,xxx and up).
2. - I find it's an issue occasionally, when it's a smaller sale (xxx)
Lately I almost never do outbound for *smaller domain sales*, until that domain is past the 60 day lock period. The exception is with Godaddy domains; if I have it at GD, almost every end user seems to use GD, or know who they are, so any domain I get through GD I feel is safe to start outbounding right away.
The above is my personal experience; others might have different results.