What helped me jump in our mtg inventory is by putting an ad on Craigslist that I buy monster boxes for $30 and pay 10 cents each for bulk rares. These are rares that only have a 50 cent, or more, value, but people will still buy them. With there being so many formats in Magic people who normally play one format will usually sell the cards they have of the other formats that they are not using. I bought a lot of cards off of some standard players who tried to get into Legacy, but didn't like is and they saw my ad for buying monster boxes and sold them to me. Got some pretty good cards. I have had gaming store selling me bulk rares and bulk cards.
We have compiled so many cards over the past year that my son is till going through them and we have a box that hold around 3,000 cards for 50 cents a card, that we have at sales, and do very well with it.
Remember, there are rare and valuable cards and there are support cards. A lot of money can be made selling the support cards.
With all of the extra cards I make starter decks that are a lot better than the ones that Wizards sells and have sold quite a few of them at $5 each. They mostly contain commons and uncommons. I'm a good deck builder.