Most buyers who are domainers will be looking for "pushes," not "transfers." If the buyer does want a transfer in the first XX days, the method that has been mentioned before is for them to create an account at the registrar in question, have the name pushed to that account, and then they can transfer the name to the registrar of their choice when the clock has run.
If you are registering domains at a registrar that most folks do not want to do business with/at, you may find it is in your best interest to move to one that people recognize and are more willing to accept a "push" from.
-Allan
