I've managed to survive as a profit-making bookseller for some years now, but not without the odd self-inflicted wound. Of course I may well be inflicting others on myself even as I write, but I haven't yet realised what they are.
Overpaying for books does not help your profit margin
As I deal in the books I like to read myself, occasionally I get carried away because I see a book and I want it myself. My most spectacular effort in this direction was a copy of J A Allen's reprint of Stubbs' The Anatomy of the Horse. The seller no sooner mentioned it on the phone than I was practically biting her hand off; offered her far too much, and realised as soon the book arrived and I had come down off my cloud of excitement at seeing the book for the first time that no way was I going to make anything on it. I didn't. I had the book hanging around for 2 years because I was desperately hoping to sell it at the top of its price range. I eventually decided that hey, maybe I should just keep the book myself, at which point it sold.
Not being precise enough on terms
I really crashed and burned with the person who sold me the Stubbs. I bought a few more non fiction titles from her. In my (then) terms, I said I would pay the postage costs for getting the books to me. What I had not envisaged is that the seller would send each and every book separately, and by Special Delivery. Of course I just had to swallow this and pay up, but I have been very, very careful ever since to specify parcel post and as few boxes as possible.
You will never sell anything if no one knows you have got it
If I catalogue regularly, I am less likely to shove boxes out of the way somewhere so I can get into my office, completely forget I have done so, and find them years later. Some of last month's new stock, I am ashamed to say, fell into this category.