I've been working on this myself. I would like to eventually NET around $1500/mo. My first month out, I did around $400.
Here's my plan:
-Find a niche. You can't be all things to all people. You need to know one or 2 areas REALLY WELL. I suggest starting with things that are interesting to you and then research that section of ebay every single day. I go into categories and sort them by price (highest prices first) so I can see what's really selling well. When you find a seller that's making good money in your area, save them as a favorite seller then visit them at least once a week to see what their offerings are.
-Do not piecemeal. I have learned the hard way, that by adding one thing here and 2 things there, that you don't make good $ at that unless it's just a rarity that everyone is looking for. For me, I'm listing purses and scrapbook mainly so I have decided each week, to work toward NEXT weeks listings. That way I have 20 things ending close to each other. Why? Well, I figure that I can advertise for all items on each auction (at least an explanation to what else you're selling and a link), and that will draw free additional advertising to those auctions. I found that out the hard way... I had 4 purses with no bids and I'd sold almost every one up to that point.... mainly because I had other related/semi-related items out there.
-Cross promotion. It seems to work better if you are selling in one or 2 areas that might be slightly related or have the same consumer base. I sell scrapbook supplies and purses. Scrapbookers are mainly women, and women carry purses. Scrapbookers also, are not scared of dropping some $$ when they really want something. They are also notorious for finding the best bargain possible

(I know, I am one).
-Bits and bats. I made a goal to go to garage sales every week and pick up 5 things I could sell on ebay. So far I've met my goal, and have found a variety of items to sell. I thought this could be a good way to test the market and see what sort of things would do well for me. So far, it's been... "ehhhhh" on those sorts of sales. (do have a bunch of printers blocks up if anyone collects em lol). Go back to "find a niche". I have found that there are a LOT of things I don't know whether I should buy or not because I'm just not sure if they will do well or not. I've come home and done research and found that most of my decisions have been good ones.
There's supposedly a couple here in my town that makes $80k a year off of ebay but I don't know them personally. If I run across them at some point, I will definitely scrape the inside of their head and get some pointers.
Personally, I don't think going out and buying huge wholesale lots that just anyone can get is going to do well. JMO of course. Before you do that, I'd research and see what everyone else is selling if they are selling the same thing. I think uniqueness is the key really. (or cheaper than the other guy).
I know many of us are trying to sell the next big "bizarre" auction, but lets face it, unless you have a "gorgeous" body or think of a new body part to advertise on (Advertise on my tail!!), it's pretty much been done to death. GP is the big player, everyone wants them, and they are only going to bid on the first and the originals. (read: copying *yawn*).
And lastly, I think it's something you need to have some sort of passion about. Sure, I could probably invest in computer widgets and sell them for good $$ if I knew enough about it to sell them, but I do get bored easily. I love creating new purses and scrapbook stuff so that keeps me doing it.