There're 2 categories: Tama user and Tama collector.
Tama users buy tamas with intention to play with their own tamas. So, they can paint and customize theirs. After when it gets worn out or lose interest in it.....its all down to their decision. The value ($$$) will go down a bit once taken out of boxes and fingers on tamas (i mean fingerprints)
Tama collectors buy tamas for collection purposes. Its to say, they don't play with tamas but to keep these in good conditions for displays (never taken out of boxes). The value ($$$) will be raise after a few years.
So, my small advice is if you are serious in value, don't paint or decorate your tamas. that's simple.
Sorry if i sound so old....its cos i'm old! haha