I have more training droids than needed to bring my whole leftovers to level 85. However, there is something called supply and demand. If they allow for selling stuff like this the price would be so incredibly low that you would have to sell half of what you piled up in order to buy a few pieces from whatever rare gear you need.
Yeah, credits have always been the bottleneck for leveling characters, at least for me. I've never once been limited by training droids. It would be nice if I could trade droids for credits, but I wouldn't count on that happening anytime soon.
I have the opposite problem. I have millions of credits but not enough training droids. I use the training droids as soon as I get them, and don't have the cash to buy crystals to buy training droids. Wish I could spend credits on training droids.
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If you had enough credits to pay for it... That's the bottle neck