It is a difficult question with many options to go for. However it has been discussed earlier. There is a topic with >10 replies, but I can’t find it at the moment.
Quick search found these topics:
http://discourse.codecombat.com/t/selling-unwanted-items/2453?u=j_f_b_m
http://discourse.codecombat.com/t/idea-for-codecombat/1675?u=j_f_b_m