Traders just take advantage of arbitrage opportunities and I don't see anything inherently wrong with it, so I believe their impact on the game is mostly neutral. That said, some of them manipulate the market to keep prices artificially high when they've stockpiled a lot of a specific resource and while that benefits the handful of other players who are also selling that resource, it hurts the masses who are trying to purchase it. Case in point: certain traders are currently inflating the price of lead by making buy offers several hundred ppu higher than the other buy offers on the market, just to keep the prices high while they sit on stockpiles they wish to liquidate. In that particular type of situation, it hurts most of the game while only benefiting a handful of traders.