/trade add undocumented feature

There was a problem with /trade add when adding unique items to the trading post.

Very occassionally some of the items would randomly change there Id and hence the incorrect items would get posted. The system later detects the corrupted item and deletes it. Alas your item is lost.

I have not been able to work out what is different about these items - but I have been able to trap it now before they get corrupted.

You will now get a message saying that you are not able to trade these items and the trade will be aborted.

This does not fix the cause of problem but at least you will not randomly lose stuff.