How does the Ripple trust system come into play here?
It doesn't. The transaction described is just like a bitcoin-otc transaction; if you send a non-reversible payment to someone first you're trusting the other party won't just go away.
That's why I think all these BTC/XRP trade offers I see on this forum are moronic (other than those looking to by a starting 300-500 XRP). The prices inside Ripple are way better and the only entity you need to trust is Bitstamp (in order to get actual BTC or USD out). I'd rather trust Bitstamp then some random dude on this forum.
Where i can chance Ripples ? Any Exchance
If you're asking about a BTC/XRP exchange look at these posts:
buy now you have his "iou" right? Just go cash it because we all know there are tons of people that like buying the debt of unknown strangers.
No. Just No. Stop spreading FUD, it's clear from your posts you have no clue how Ripple works nor do you care to learn or contribute constructively.