In Lawrenceville try 'Books By the Pound'. Gave me $18 credit for a bag full of non-fiction.
Half price books does buy used books but the trade in price is not very consistent when you compare it to what the average resale price online. I am not sure what scale they use but most people complain that their are not fair with there resell buy back prices.
Half Price Books does purchase used books, however it has to be a book that they may want to add to their collection.
Thanks! Your answer is awaiting moderation.