Buying from garage sales and selling on eBay is fun, and good getting a few dollars here and there from ebay. Sometimes, you luck out on an item and hit big, but many times it will just be so-so (assuming you can even get the price you paid for it). In terms of fun factor, it is great. But from a business stand-point, it is not. Not sustainable.
If you want to earn serious money from eBay and get rich from ebay, you need to have a well-thought out business model. Selling on ebay is getting tougher — just look at the number of auctions that closes without a bid. Go to the boards on ebay and you will hear even oldtimers complaining about how stiffer competition plus higher fees make it so much harder to sell on ebay.
One success model of ebay is the couple Jay and Marie Senese (username jayandmarie) of the 1-cent CD fame. They have the highest positive feedback number of all sellers on eBay. Their business model is to buy very low cost CDs in bulk (those that CD stores sell in their discount bins) and offer them on ebay at a starting price of 1 cent Their success is based on the fact that they stumbled on a sustainable business model with products that people really want. The couple sold their business for $5 million — where else, but on eBay.