how do you

[planterhr12]

how do you become a vendor, for example an avatar vendor

[Rolig Loon]

Anyone can sell anything that s/he has transfer permission on.  There';s no magic to it at all.

If you want to sell things in world >>> http://community.secondlife.com/t5/English-Knowledge-Base/Selling-objects/ta-p/700143

If you want to sell in Marketplace >>> http://community.secondlife.com/t5/English-Knowledge-Base/Selling-in-the-Marketplace/ta-p/700193

There's more to it than just the mechanics, of course.  Anyone can be a merchant, but you can only be a successful merchant if you put some energy into marketing your products well.  THAT is an art.  The guidelines here were written for merchants in Marketplace, but they really apply anywhere.