Borderlands: Game of the Year Edition

This trophy is cumulative and you don't need to have the entire $1 million on you at once. Pick up and sell everything you don't need and you will have this in no time. This trophy can also be done in conjunction with Fence. You should easily obtain this in your first Playthrough.

Credit to OnceFallen for pointing out the following:

The absolute fastest way to get this is have a friend with a Max level character give you an item e.g. a class mode that's worth a million or more, then all you have to do is sell it and the trophy will pop shortly afterwards. Use this trick if for some reason you don't have $1 million after obtaining most of the trophies or if the trophy for some reason doesn't unlock.

Manifest Density Pt. 2 has pointed out the following:

Create your main character and play a couple missions, save exit. Now exit out and create another character, go into online and join a game that is way further along in the story. You'll rank up pretty quick, gain a couple extra weapon slots etc. but the key is to pick up all the weapons of value (anything over $5,000 resale value, though anything over $12,345 & $123,456 is even better to pick up).

Now after you fill all your backpack slots, save the game, exit. Go into splitscreen and have P1 be the MAIN character and have player 2 select your name and pick the character you went online and collected the weapons. now drop everything you collected and let P1 pick them up, sell them and bank on them. Let the game save, exit and go back online with the 2nd character again to collect more weapons of value to do it again.

The Reason for doing this with the 2nd character is so you don't end up getting the mission glitches on your main game. I hope this helps anyone who is looking or needing some cash early on.