Actually, to my mind, it's more like buying the book, reading it, then returning it. (I know a guy who does that.)
What a local retailer offers is the ability to look at the merchandise, try it out, see if it works for you, plus the advice on whether it's right for what you want to use it for. The retailer bundles that service into the (higher) price for the product than you'd pay from some e-tailer.
For you to deliberately take that service without the intention to pay for it is, in fact, theft. Perhaps theft of a service (or an implied service) isn't technically shoplifting (my attorney wife is out of town, and these sorts of discussions annoy her anyhow) and couldn't (and shouldn't) be prosecuted, but it certainly is morally wrong.
To me it's like having a book store employee read you the book, recommend others like it, then you walk out.