you are not legally obliged, unless your signature is on a peice of paper. If you don't want to purchase something, thats your right.
But, what I will agree with hands down is don't flake and just dissappear, explain yourself. IE. Unavailable funds!... Things come up you know. But to just disappear when the seller is sitting there on his hands telling everyone its sold, when if you come out say "Look, I just can't buy it, I'm sorry to trouble you." The worst is that the seller is a little let down that now he has to put it back up, but he's not sitting there for days/weeks when they could had gone back to their thread and say. "Sorry, the deal was called off, item back for sale, any new takers?"
Now, on to the "pre-order" now there is the only part where I'll agree with "obliged." Because you are getting someone to pay for something, instead of displaying interest in an item that is already in stock. The only thing is still, no signature, so your best bet is to do the deposite, they you have a legal lock with them. And they already paid half! Why pull out now?