Tips and Recommendations for when buying online?

[DragonMasterX]

It's my first time doing something like this and before I actually go through with it, I'd like to know if some of the more experienced users on here have anything to tell me that I might not already know about taking necessary precautions before commiting to pay for something that an online seller will send me.

I realize things will be different since I'm gonna be using MercadoLibre.com (Argentina's eBay), but I think the operations should retain some similarities since it's the same concept...

So, what should I know before, during and after an online transaction?

(Btw, the article I'm buying costs 150 pesos, which is around 35 or so dollars, just for your info)