The PlayStation3 that I want is a very particular model, so it's an important thing to have it identified correctly. Here's a few pointers that will show you what to look for when buying it.
The first (and most important) thing to look for is the model number. On this particular PS3, the model number is CECHAO1. You can find this by looking on the information sticker on the back of the console, on the bottom of it. Here is an example:
Another sign you can look for is the memory card readers. On the models after 2nd generation, there are no memory card readers. When looking at the console, to the left of the CD drive there is a small black section. On older models, you can lift this section up to reveal the flash memory card reader slots. Here is an example:
The last major identifier is to look for the size of the hard drive. By default, this model comes with a 60GB hard drive. However, as the hard drive can be upgraded, this may not be entirely accurate. Most people don't upgrade the hard drive though, so it's usually a safe bet. You would still need to check the model number on the back to make absolutely sure. This will probably be the easiest thing to search for on the internet in order to find it. (i.e. search "60GB PS3" and this is likely the correct model, yet you still need to check the model number or have the seller tell you)