To unlock the Better Than Money trophy in Borderlands 2, you will need to purchase five items from the black market.
In order to purchase items from the black market, you will need to complete the steps below (taken from the Xbox 360 guide by AHGreg):
1) Find Sanctuary - Story Mission
2) Complete the first mission given by Scooter
3) Then after that Crazy Earl should start appearing next to Mad Moxxi's place
4) To buy upgrades you must pay in Eridian (purple crystals)
5) To find Eridian crystals you can either get them from completing missions, scavenging them from enemies or looting them out of boxes/crates/lockers/etc