Depending on what you stumble across, Half Price Books can be a steal or a bust. Unfortunately there's no way to predict how your trip will go ahead of time, but that can actually be fun if you're looking at killing some time while browsing.
There are some good finds in the clearance section, and with some patience you can find some great deals. Two dollars for Fleetwood Mac's Rumours on vinyl? Okay, I'll take it. My boyfriend has found some comic books priced at a fraction of their actual value due to his encyclopedic knowledge of several series.
HPB stays at three stars though for their overall pricing and buying. I find that many books I'm interested in are priced almost as high as their list price, which seems silly for a used book. Prior to a vacation I came in to find something I could read and then leave behind, but at seven to ten dollars for a paperback I might as well just buy something new. What I've been offered for buys is also on the extremely low end considering what they're going to turn around and price the books at. I brought in several best-selling hardcovers for my mom that were in nearly new condition (you'd never know someone had touched them aside from finding them in a resale shop) and I got less than $5 for items that would retail for $200+ new, and at least $100 in their store if priced like the rest of their inventory. I understand there are several factors at play, but the time spent waiting for the offer and tiny payout make me more likely to just donate them to a library.