Also, there is the question of packaging. If I take my existing shopping bags to, say, Foyles, and buy a load of books, then I am not using any extra packaging. If I send Richard to buy them, then we're not using any more resources at all, because he can walk from his workplace to the bookshop, so there's not even the issue of transport. Whereas, if I buy the books preowned, each book will come in its own jiffy bag that I'll then end up storing along with the 100 or so other jiffy bags that are in this house waiting until they can be reused, which I can't throw away because of the environmental guilt and can't recycle because this country is not that efficient yet. They'll also have a packing slip and/or invoice which can't be recycled because it contains my name and address information, and while I often do rip off that part to go in the shredder while recycling the rest of the page, this takes Effort and spoons. Most likely, they'll pile up until I have energy to deal with it.
Of course, the sensible answer would be for me to go to a bookshop to buy the books second-hand, but there aren't physical bookshops that sell chemistry textbooks. You're stuck with physical bookshops that sell new or online bookshops that sell used. Occasionally, universities have a second-hand textbook fair, but they tend to be limited to students of that particular institution, and wouldn't be advertised externally. Similarly, another idea would be for me to borrow the books from a library - but public libraries don't carry university-level science textbooks because there just isn't the demand for them, and I don't have access to a university library. I could get access, but that would be to go there and study from the books there, rather than a borrower's pass - those are pretty much impossible to come by for outsiders. And I don't like to sit in libraries and breathe their dust and photocopier fumes and deal with chairs that aren't designed for my back when I could take a book home to read.
Considering that we're talking about small paperbacks of around 100 pages each, and considering that these books have already been printed and are sitting in the bookshop waiting for someone to buy them, I'm inclined to think that buying them directly with my existing bags might actually be the most environmental of the options available. But it doesn't seem very satisfactory.