Start small and grow with it. My first collection was bought some 7 years ago.. I spent $25 on it and sold it for $100.. then I bought a $100 collection and flipped for $300. Eventually I found that tripling my investment money is THE LEAST amount I would expect to gain from a purchase. Sometimes it would be much much higher, but rarely would it be lower. After establishing credible sales continuously, I implemented another rule: never spend more than a 1/3 from the profits of the previous month sales. So if I earned $4500 in sales in February, I would set a budget of UP TO $1500 for March. And of course I would carry over any amounts to following months if applicable.
I managed to acquire hundreds of collections in just a few years. My selling objectives are selling current trend books and keeping established keys for retirement. You can afford to do just that when you have a collection this size assuming it's diversified and you spent ample time organizing it.
Everything else just falls into place logically, it took me a couple of years until I decided to grade my books, as well as gain knowledge into what to grade and what not to grade. But the most important thing for me is to always budget it appropriately first.
Good luck