CovrPrice is one of the best known standard crawlers out there - with a recent partnership with the most widely used collection logging app (CLZ), it's popularity has skyrocketted.

It's modern, well-designed, and attractive... Though some of the prices are wildly inaccurate. CovrPrice has its own crawlers, that fetches tonnes and tonnes of information, though when those crawlers hit a lesser known issue and stumble across a random sale that may or may not be verifiable, it returns that value and sets that issue at an insane price. For example, when CLZ announced the integration, I was hyped. I bought it instantly, then deleted it instantly. 

Why? It was marking 20p books with a value in the thousands of pounds, and completely skewing my overall total. Keep in mind, this was on the day of release which was likely over a year ago now, so the kinks may have been ironed out since.

Regardless of that fact, CovrPrice is a good website, though it pushes pretty heavily for paid memberships.

Pros

- Reputable and well known

- Easy to use

- Attractive with great UI

Cons

- Can be inaccurate

Check them out here