When it comes to buying your favorite team's jersey, you have two options. You can get official gear from a licensed shop such as Soccer.com, or you can look for a slightly cheaper knockoff on eBay.

Being the world's largest online marketplace
eBay has an enormous selection of replica soccer jerseys. Chances are, you can find any jersey you want, in the size you want, oftentimes at a bargain price. Keep in mind however, that many of the shirts are sold by merchants from Asia, making for slightly higher shipping prices and longer shipping times. This also raises some questions about the authenticity of the product, as many of the countries in the region are lax in their copyright infringement enforcement.

In the end- there isn't a right or wrong answer. You can choose an authentic replica jersey and wear it the way it was meant to be worn, or choose the cheaper alternative and save a few bucks on a shirt that looks and feels just like the original.

Sizing is another point of consideration when shopping online.  European jerseys usually run one size up from traditional American sizes, so a European Large fits more like an American Medium, and so on. The best way to figure out the right size for you is to get the actual measurements of the jersey (length and chest), and compare it with a shirt you already have.

Another thing to keep in mind is that soccer clubs and national teams periodically update the design of their jersey, usually updating a few small details, or to change the corporate logo.  All of the big soccer shops linked to by this site update their shirts yearly, or whenever new editions are released.