If a deal is too good to be true, then it probably is. Developers charge a good amount of money for their themes, say 40$. If anyone sells it for 10$, then most of the time it will not be legal. However, if the person has resell rights then they are probably allowed to do so. To be on the safe side however, I like to buy themes and plugins directly from sites like Codecanyon or Themeforest. I would recommend Elegantthemes as well because the themes can be compared to almost every other expensive theme (30$+) however the price is "dirt" cheap, because you pay only 89$ a year for over 80 great themes and some useful plugins you might need.