I have | I believe the price in USD goes | I want to earn | I should do the following: |
BTC | UP UP UP | BTC | Borrow USD (you need some USD to do that!), buy BTC now (you think they are cheap!), after the price goes up you sell enough BTC for USD to settle the loan + interest, keep remaining BTC |
BTC | UP UP UP | USD | Borrow USD (you need some USD to do that!), buy BTC now (you think they are cheap!), after the price goes up sell all the BTC you bought for USD to settle the loan + interest, keep remaining USD |
BTC | DOWN DOWN DOWN | BTC | Borrow more BTC, sell them for USD now (you think they are overpriced!), after the price goes down, you buy the BTC back with the USD you gained, pay back the loan + interest, keep remaining BTC |
BTC | DOWN DOWN DOWN | USD | Borrow more BTC, sell them for USD now (you think they are overpriced!), after the price goes down, you buy just enough BTC back with the USD you gained, pay back the loan + interest, keep remaining USD |
USD | UP UP UP | BTC | Borrow more USD, buy BTC now (you think they are cheap!), after the price goes up you sell enough BTC for USD to settle the loan + interest, keep remaining BTC |
USD | UP UP UP | USD | Borrow more USD, buy BTC now (you think they are cheap!), after the price goes up sell all the BTC you bought for USD to settle the loan + interest, keep remaining USD |
USD | DOWN DOWN DOWN | BTC | Borrow BTC (you need some BTC to do that!), sell them for USD now (you think they are overpriced!), after the price goes down, you buy the BTC back with the USD you gained, pay back the loan + interest, keep remaining BTC |
USD | DOWN DOWN DOWN | USD | Borrow BTC (you need some BTC to do that!), sell them for USD now (you think they are overpriced!), after the price goes down, you buy just enough BTC back with the USD you gained, pay back the loan + interest, keep remaining USD |
The problem is that if you have 0 USD but want to borrow USD, you need to have at least a few USD to start with - since the market might move in the other direction, you need to be able to pay the interest and buy back the USD or BTC you borrowed at a loss. Same way with having no BTC and wanting to borrow them. There is no way to transfer funds between accounts on Bitfinex (yet?), so the easiest/fastest way to get a balance on both sides would be to either use MtGox codes (USD codes will go away soon!) or just transfer BTC on Bitfinex and trade them for a few USD, the fees are lower than a lot of other exchanges anyways. In the future bank transfers might also work, though BTC+trade probably beats these in speed by a magnitude.
If you want to lend out funds, you also see when you'll make profits: People will borrow your USD if they think that 1 BTC will be worth more USD than now in the future and they will borrow your BTC if they think prices will fall. Believing prices rise is also called "bullish", believing prices go down "bearish".
PS: Someone seems to just have borrowed ~400 BTC (mine amongst them), so I'm happy.