Well good old E-bay I guess everyone knows the most famous internet auction site, if you have been living in a cave it is a site where people place items they wish to sell along side a photo (usually) and a description and people bid how much they want to pay for the item. They can also sell at a set price called "Buy it Now".
Now whilst the prices intially look good you have to add in the cost of postage and packaging which varys wildly between sellers, be aware with some low cost second hand items (and others) this is how the sellers makes the bulk of there money.
I have both bought and sold numerous items on E bay (bought more though don't tell the missus) and have found it to be a useful place to obtain items. The main items I have bought are books, cds and dvds, warhammer (a model based game), and clothing and to a lesser degree these are also the items I have sold.
To give you some reassurance each buyer and seller has a rating given by previous buyers/sellers which will give you some indication they are trustworthy.

Pro's

You can get some bargains if you search about, items ending at unusual time or bad times, items in the wrong section or misspelt or even just items with low bidding prices or bulk buys can all yeild excellent bargains.

Con's

Do your research on the internet can you buy it new cheaper? remember to add on the sellers postage and packing cost, you can also get into a bidding war I recommend pick a maximum price your willing to go to and stick to it