If you have ever studied about forestry, horticulture, plant breeding or anything that’s even distantly related to plants; you’ve studied a part of agricultural sciences. This field is diverse, as you might have already figured out, and combines so many different disciplines that you’ll be able to sell a large number of your old books to someone who studies agriculture.
Students of agricultural universities find their use in various industries, from being a simple farmer to working as a trader on the stock market, these guys are everywhere. This means that there are a large number of people out there who would love to buy your old books, no matter the age. Some, however, might only be interested in renting them, but this would only create a revenue stream for you.
As in today's world, we require higher yield from our crops, the demand for those who've studied agricultural sciences is forever increasing. You can capitalize on this demand and sell your old books faster, as there are a lot of students and professionals looking forward to buying them. Moreover, some of these individuals in future might be able to develop some revolutionary crop varieties and your old books would have an immense contribution in that. So, I would conclude by saying, sell to feed others.