I've had international shipments take a month or more to make it to their destination before so if they've only been waiting two weeks and they're from the US, they haven't waited long enough.
Second, it is the responsibility of the sender to provide proof that they have sent their part, whether it is a tracking number, customs number, or something similar to that. If they can't provide any proof that they sent their part, there is nothing you can do but wait and see if their part arrives. Personally, I wouldn't submit a bad traded report for an international trader until maybe 6 weeks after they sent their part, and even then only if I really believe that they are being malicious.
Third, since you are often doing international trades, you need to be extra careful about who you trade with. You need to make sure that you trade with people who understand and are willing to wait the extra time that is sometimes necessary when shipping overseas. If someone is telling you off after two weeks for an international trade, that's probably someone you don't want to trade with again. There are fairly young children who use this website and they may not understand that it takes time for international shipments to make it to their destination.
Overall, try not to worry too much, you haven't done anything wrong!