https://github.com/RegalOwl/HyperConomy/blob/424200b94fd2fb57364a5f36e927ee040e2d480b/src/main/java/regalowl/hyperconomy/HyperAPI.java#L265
and https://github.com/RegalOwl/HyperConomy/blob/424200b94fd2fb57364a5f36e927ee040e2d480b/src/main/java/regalowl/hyperconomy/HyperAPI.java#L284

Seems to be using the wrong method for finding the available TradeObjects.
My Guess:
if (availableObjects.get(i).getStock() != -1) {
should have been the correct access.
Although I don't really see, what this loop should do, it does not work correctly in the current way. (TradeObject and int NEVER equal, so the return value is ALWAYS -1)