From a purely technical standpoint, with no tracking, anyone could claim that your cards never arrived. To this end, much of the trading done on here is done on the honor system. However, as stated in the trading rules section, you'll note that it is recommended that you use tracking if you're ever in doubt as it is the sender's responsibility to get the cards to the other party - with any luck, they'll be upstanding enough to be honest about whether they received them or not (also a good reason to typically deal with people that have existing rep if possible).
Now, speaking from personal experience, you really are just taking a risk. Thankfully every trade I've made has arrived safely, tracking or no, but that's not to say that something couldn't have been lost in the mail. If it's a small trade (say less than $10), most people will send it without tracking with the knowledge that it could be lost and they're just out of luck (such is the price of doing business this way I guess). For anything over say $30, I would say to ALWAYS use tracking as, if nothing else, that covers your rear in the event something gets lost.
Sorry to hear something got lost... but odds are if you maintain clear communication with your trading partner, you'll be able to work something out.