Ebay still might be worth a look, even if only to get ideas and brands...it assembles a lot of products in one location. (I've had great success on ebay, but I understand your hesitation)
I was given one of these:
http://www.targus.com/us/productdetail.aspx?regionId=7&sku=TCG200&PageName=Messenger Bags for Laptops by Targus&productCategoryId=6&bucketTypeId=0&searchedTerms=&navlevel1=shop&cp=&bannertxt=Messenger Bags for Laptops
(That whole thing is the link...I don't know how to use the link feature here)
Which was big enough for my 17" Dell and two texts (thick ones, like the Torts text by Prosser). But with two texts, I usually had to carry my portfolio and any supplmenets. That bag was big enough most of the time, though, and it was really durable. It certainly isn't the most stylish bag, but it's not awful, either. It worked well with a peacoat. Another plus is that it handled winter snow very well and the strap could be adjusted easily. When it got a little soiled from sitting on the floor, a quick mix of detergent and water applied with a toothbrush cleaned it very well.
Personally, I don't like traditional black leather bag...theyr'e too commonplace.
Just my thoughts...I've been in your shoes, that's what worked for me.
jag
(PS, sell your textbooks on Amazon instead of back to teh bookstore...you'll double what the bookstore will give you.)