Avito works in exactly the same way as other similar apps, such as Wallapop or letgo. In other words, you use the search tool to find exactly what you want to buy, and once you've found it, all you need to do is to negotiate with the seller. In the description of each article you can find information including the price, seller's location, if they ship, etc.
Avito is a particularly appealing buying and selling app for Russian residents. Users who do NOT live in Russia or don't understand Russian, will of course find it difficult to get any use out of the app.
Requires Android 4.0.3 or higher