A first printing Nancy Drew sold at eBay recently - The Mystery at Lilac Inn - for $4302.22. Pricey, yes, and would have to be priced up more to resell. Let's break it down.
Condition: DJ is very poor - see all the water staining and possible mildew? Unfortunately the seller didn't show the full DJ so I'm not sure how much wear and tear it has. But these kind of flaws devalue a book greatly. Why? We'll, it's got imperfections--in this case huge imperfections. Plus, if one were to invest this much in a book of this condition, in my opinion, it's an expensive upgrade. You're always going to be wanting to upgrade this, so why not invest your money in one in better condition? This, as a general rule, is how I make my purchase choices--it's easier to resell a book in better condition if you end up upgrading. So, it's risky.
The book is another story--look at how chewed up it is. Definitely due to the water/moisture damage in part, but there could be mildew. Mildew spreads to other books, though there are ways to try and get rid of it. I don't have a first printing of this book in my collection, but for the above reasons, this is why I avoided this auction, that and the high price.
Farah's 12th Guide to the Nancy Drews values the DJ at 3500.00--that price, however, is for a book in very good condition, little flaws. So this book sold well above guide and condition value.