Basically, readers found the books, enjoyed them and recommended them to their friends. E L James had built up a following via the fan fiction site, but if the novel hadn't provided what a lot of people wanted, it wouldn't have sold. Simple as that...
Of course if no one knows the book exists, no one can recommend it to their friends, so you do need that initial starting point, but if people don't want to buy something, no amount of advertising will make it sell.
When we ran our small educational computer software company in the 1980s, we had a few best sellers. We tried to analyse what made them popular because obviously we hoped we could repeat that success with subsequent programs. Unfortunately this wasn't a simple matter because our three big sellers sold for slightly different reasons, but in each case, the program provided exactly what teachers wanted or needed to make their teaching easier. We experimented with marketing, trying to increase sales of the less popular titles, but in the end we discovered (as publishers of books had no doubt already discovered before us) that to maximise sales you push the things that are already popular and putting loads of effort behind the slow sellers doesn't turn them into hits.
There were some things that made a difference. Whichever program was featured on the front cover of our brochure always got a boost in sales. In fact people obviously weren't reading to the back because having chosen to feature an old program on the front (we used to rotate titles to boost sales), we often got orders for, "Your new program on X." This was despite the fact that it might have been available for over 12 months and featured in numerous mail shots, just not on the front page.
The recommended strategy for generating sales these days is to have an effective online presence, but are a lot of writers wasting time on blogging, tweeting, setting up Facebook pages and so on and so forth? Probably. What seems to be the best way of getting people to buy your book? Just write a book that lots of people want to read and let at least some of them know that it exists. With luck, word of mouth will do the rest. But with all the millions of blogs, Twitter, Facebook and Tumblr accounts out there, how do I catch the attention of the people who might like my book? Heaven knows!
[Cross-posted from Dreamwidth by way of a backup http://heleninwales.dreamwidth.org/60968.html. If you want to leave a comment, please use whichever site you find most convenient. Comments so far: .]