But, how do you know if you're paying a fair price, getting a bargain, or paying five times as much as you should?

The easiest way is to do a quick search on Bookfinder, at www.bookfinder.com, for other copies. By studying the listings you should be able to come up with an average price for the book, based on its condition. You might even find a cheaper source.

Sites and sellers from all over the world are indexed by Bookfinder, although the majority of the books listed are from the United States. The next most common is the United Kingdom, with most of the remaining sellers being based in other English-speaking countries. Entries from Amazon are listed, along with ABE, Alibris, and other less-known sites.

If the book you want is available cheaply from another country, it's still worth considering. Do check the shipping charges and factor them into the price, though. If the book costs more than $28 (US), you could also be charged import taxes. However, in practice, this only tends to happen with high-value collectibles.

On the subject of buying abroad, it's also worth checking other Amazon sites around the world. Unlike eBay (tm), each listing by a particular seller on Amazon is shown on one site only, so you'll only find the bargains if you shop around.

Speaking of eBay (tm), it can also be a good place to buy books. However, because the site isn't as specialized as the others I've mentioned, you might have to wait a long time for the book you want. It's always worth a look though, as there are bargains to be had.

I frequently compare Amazon.com, Amazon.ca and Amazon.co.uk when buying used books. It's not unusual for the prices to be so far apart that it's possible to buy books one, to sell on the other for a handsome profit - even after paying for the extra shipping. You don't need to open a fresh Amazon account for each of the sites - your current login details will do. You can also use your home delivery address and credit / debit card.

In addition to the condition of available books, supply and demand are very important factors, and account for price variations from one site to another.

If Amazon Marketplace has only one copy of a particular title then the price will usually be high - especially if the seller is a professional dealer. Other sellers come along, and get a false sense of the book's value. They may undercut the first seller, but not by much. This establishes an inflated baseline price.

Sometimes they actually get the asking price, especially when the book is hard to find and the buyer is desperate for a copy. More often though, the books languish on Amazon until someone is brave enough to reduce their price. It's worth contacting sellers and making an offer. Amazon sellers are often happy to accept a lower price when a book remains unsold after several months.

By using a meta-search site, such as Bookfinder, you'll be able to check out the prices a book is being sold for, across all of the popular sites - and in several countries. This will help you find a much more realistic value than just looking in one place.

Bookfinder is also very handy if you're planning to sell a book online. It makes sense to get an idea of the price you can expect before placing a listing. If you set too low a price, you might get a quick sale but you'll reduce your profits - too high, and your stock won't turn over quickly enough to make your business viable.

In case you're wondering, I have no affiliation to Bookfinder. I just find the site incredibly useful, both as a bookseller and a book lover. Once you've tried the site, you'll wonder how you ever got by without it.