I never list any of the salient aspects of the gear that I am selling, other than condition. The reason, simple, after many, many transactions, if someone has not done the research to know what they are buying, I really don't want to sell to them. I am a hobbyist not a retailer.
Now, you could make the argument that they are going to see that I am selling a 400 watt amp and then figure out if it is what they need, but my practial experience runs counter to this. BTW, IMHO the input impedance of an amp or the output impedance of a preamp, is going to be a much more salient bit of info than how many watts an amp puts out.
So yes, in my estimation it is too much to ask. And I understand that may preclude us from doing business, but it seems that we are both comfortable with that.