1) Don't generate jar files while testing a script, you should do that in the end, otherwise you must restart etc. What I do, is just make your project folder on Users/OSBot, and change your output folder to OSBot/scripts, so all the .class goes there and you can just save script on your ide, and refresh scripts on OSBot and you don't have to restart client every time you make an update
2) A trade request is a message, so you must add a messagelistener to your script by doing the following; put this in your onStart method:
getBot().addMessageListener(this);
and then add the overriden method onMessage(Message m) the message will be
String msg = m.getMessage();
and to check if its a trade, just do m.getMessageType(), which is an enum and contains RECEIVE_TRADE (in your case)
As for the last question I've never handled trading in a script so I'm unable to assist you there, but there's an open source merching bot (which handles trades) in the snippet section iirc.