I like the idea of easy registration, but I am not really sure that No Registration Required is all that attractive to me (as a buyer or seller) for this type of application.

Other user goods marketplaces give an idea of seller's histories, in form of ratings, reviews, and so forth. I think that can be really helpful (and I do tend to avoid Amazon marketplace sellers with low ratings). Is that a feature you will want to provide?

Currently book listings have a Contact Seller link. I have not yet come across multiple listings for the same book (different sellers). How will you effectively distinguish those listings from each other?

Some smaller items that stood out:

http://beta.mrtextbooks.com/browse (Buy link) is broken. On Buy|By ISBN,By Author, By Title all links point to the same search page. Each time though the Title filter is selected. http://beta.mrtextbooks.com/search - if no matches are found (which could be due to filtering or simple title mismatches), then the same form is displayed again. It would be really helpful if you showed some message indicating "No results were found - try broadening your search" or something to that effect. Otherwise it may be hard to tell if search is simply not working or if there just aren't any matches for a given search.