Are Piano, Violin, etc.  type books that have writing in them postable?  Do they fall under the texts books category?

I bet they would fall under the textbook category...I have degrees in music and purchasing good editions were a requirement for my private lessons.  I also teach piano and they are required books for the lessons I teach too.  I would post and pmail the person about the writing as per the textbook clause.

And just curious, what do you have?  I'm willing to take them off your hands if they are things I could use in my teaching :).

Any book can be posted under the textbook exception because the receiver must be informed of and accept any writing or highlighting so they will know ahead of time and won't have someone sneaking it through. If the requester says no, you would need to cancel.

Yes Janice, if there is writing on text pages you should post them under the textbook clause.  Also check the pamphlet guidelines as some of the smaller music sheets could fall into that catagory also.