As everyone has said liability and industry protocol are essentially the same for both.
I do some pre list insp, not too many because the calls don't come in for it that much. I know that some realtors in our area shy clients away from pre list insp on the basis that 'if you don't know about it, you don't have to disclose it'. I think it's the more experienced realtors who are ok with prelist insp because they understand that it can make the deal smoother. (if the client actually fixes stuff before list)
I approach both insp the same. I do, as Ted, charge more for prelist insp. They are more work and take longer. Not only am I doing an HI but I am also, to a 'small' extent, providing consulting services. I'm not just telling them they should consider replacing the 12yr old HWT but also that they need to pack up all the crap in the garage so it looks like 2 cars actually fit in there.
One of the big problems people face when attempting to sell their home is appearance. The stains in the carpet, the nicks on the walls, the clutter in the rooms and garage, etc. These are everyday defects that the residents have become blind to overtime. I've pointed things like that out to people and they are surprised.
A prelist insp (at least for me) involves some handholding and guidance to the client.
If you don't write up the bad furnace, people are going to be pissed. If the buyers insp finds an outlet not working that you missed, it's not going to make or break the deal or your reputation.

__________________
Accurate Inspections & Consulting, Inc.
847/736-9975
Markus Keller