reserve means bidding has to reach so high in order for it to be sold. If you want to get $900 for it, but dont want to start the bidding that high, you set the reserve. But in that case, the bidding can go higher... depends on who wants it. Putting a buy it now means you'll accept that offer and once someone buys it... the auction is over.

Its taking a gamble. If you have something people want, then set a reserve to what you'll take. If you have something that is "a good deal", then set a BIN. Like my laptop i sold, it is one that people like to have.... so i knew there would be people bidding it out. But if i had a regular old laptop and i just wanted to get rid of it, i would have set a BIN with a price that i wanted for it... but not a top dollar price.