We are a couple of weeks out from Book Con and that means that I need to start thinking about what I might take to sell this year. Although it’s a conference about fiction for girls, it doesn’t just have to be books that fit that genre – last time I got some British Library Crime Classics in the sale for example. And we all know I’m always running out of shelf space so I’ve got my name down to sell a few and now I just need to figure out what, how much I’m going to charge and how people are going to pay me. Wish me luck.
Verity Reads Books (lots of them) 
My suggestion, Verity. Sell books that you can live without, at least for now, that are IN PRINT and/or NOT old and rare and expensive.
You can easily buy new copies of books that are readily available.
But OLD, and OUT-OF-PRINT books, although they are more valuable to SELL, are also VERY COSTLY to buy.
Let your valued collection grow.
Meanwhile, keep a good archive of authors and titles, so you know which books you HAVE OWNED, and avoid getting to the situation where you KNOW you read a pretty OK book about a girl who bred hedgehogs / knitted bread / fought at Gettysburg, and you THINK it was pink / sienna / had a picture of aardvarks on the cover but you can’t remember the author or title.
This is really good advice. I usually get rid of the stuff I can’t imagine wanting to read again and that wasn’t hard to acquire. I’ve got a few things that I’ve bought cheap, read and not enjoyed – and that are in the Book Con wheelhouse so they can go. I don’t need much money for them, I just know that i’ve got the best chance of offloading them to a home that wants them there as they’re not the sort of book that Oxfam books or similar is going to want or be able to sell!