Take into account current position when proposing trades.

[clesaege]

Currently, the interface only proposes to trade your items tokens for UP/DOWN tokens. But people may want to trade initially and then either change their estimates, or trade again due to price movements (ex: market estimate is 70%, your own is 60%, so you buy down tokens, but a day later, markets moves to 50%, now even if your estimate is still the same, you want to sell down tokens to buy UP tokens).