My concern about online appraisals is the potential for "malpractice" of a sort. Someone may come online asking for appraisal of an item and give incomplete (or worse deceptive) pictures of the item. You give an approximate value, but what you are valuing are pictures on a screen. A case in point is a 28 inch motobike I bought a few years ago. It was pictured from every angle except the non-drive side, which of course is where there was a significant kink in the frame from a collision with a car. Another case - an English 3-speed someone sent me in an email with lots of pictures, well, lots of pictures except where there the frame had a rust hole in the seat stay underside by the bottom bracket. Those defects fundamentally change the value, and you don't want the seller spouting off that "Mike knows these bikes and said it's worth THIS much", when in reality, it's worth nowhere near that because what you are "valuing" and what actually exists are not the same. You don't want your name tied up with this sort of behavior.
So you could give an appraisal, but the appraisal is only as good or as honest as the item presented. That's not to say NEVER give an appraisal, but they should be very sparingly given because you want it to be right.