It's a little more complicated than that. For the national chains, 70% sell-through is generally regarded as the floor for "good sales." Much less than that and it's clear the publisher, rep, and buyer agreed on too high an initial order, and the publisher will then suffer substantial returns (more on those in a future post). 80% sell-through is very good. Once you get toward 90% and higher, however, the tide turns; as sell-through approaches 100%, it becomes clear that demand is outstripping supply, and it's likely that a lot of sales are lost to other retailers (other chains, independents, Amazon, &c) because the account is out of stock. In this case, the publisher, rep, and buyer agreed on too low an initial order.
(Keep in mind that for most debut authors, the majority of stores at a given account rarely have more than one or two copies on-hand at any time. There are about 500 Borders stores and about 700 B&N stores nationwide.)
Someone recently asked me why it's such a disaster for a book to have too low an initial. (Having too high an initial is clearly a problem—the publisher will have to take the books back.) Too low can be worse, however, due to missed sales; it takes a significant amount of time for frontlist reorders to be processed, shipped to an account's warehouses, distributed from the warehouses to the stores, unpacked at the stores, and displayed. Without a high enough initial, stores and warehouses will often be out of stock during this time, and while some customers will order the book for future pick-up, the vast majority will simply buy it elsewhere.
The sell-through game is a little different for on-line retailers like Amazon, for two main reasons:
1. Amazon doesn't need to order more copies than it plans to sell. The chains have to do this in order to have enough copies in-store to attract consumers' attention, especially if the book is in promotion. Have you ever noticed how those front-of-store tables always have thirty copies of the new Janet Evanovich, no matter how many copies the store sells?
2. Amazon's search function is designed to guide you to the hardcover edition of a book even if the mass market or trade paper is already on sale. Whereas the national chains will return any hardcover copies of a given title to the publisher by about a week before the trade paper or mass market edition goes on sale, Amazon hangs onto them because they know they can continue to sell them. (They also have great discounting, so even if consumers find both the hardcover and the trade paper/mass market, the price difference may be so insignificant that they opt for the hardcover anyway.)
For these reasons, Amazon's sell-through is terrifically efficient: over 90%.
So there you have it: sell-through needs to be good (hopefully at least 70%) but not too good (90%+, Amazon being an exception). There's no such thing as having sales that are too high, but it's very possible to have sell-through that is.