[Lucid:]

Serra, commons are the items that are found in the shops for gold and never go away. so any item you can buy from Eclectic Trends, Lavish Luxuries, Threads, or Trinkets & Tresses is fair game. :3
another way to tell is when you're editing your avatar, all the commons get sorted into their proper category tabs while special/donation/event items go in those tabs only. don't touch those three tabs and you're also safe.