Hey Binx,
Some awesome replies thus far, although as you may already be aware this is an extremely niche market and has a ‘quirky’ nature to it across the board… It is certainly something worth pursuing, based on your interests, and can be run along side a full time job.
Its an area i know tonnes about to be frank, and there are obstacles in your way at most turns… Dont be discouraged though, just look for the way to overcome them..
From your post, a few things pop to mind which would assist in the responses such as:
a. State/City or close to if not wanting to disclose
b. Type of sports cards (AFL/Cricket or USA NBA etc?
Another query would be related to your existing hobby aspect of it, such as:
1. What do you collect?
2. How big is your collection (do you have cards that are desired)?
3. Do you already attend trade fairs and people ‘know’ you and would trust you entering this market as a seller?
My reason for asking is that it can sometimes take many many years to get clients to trust you in this field and it is due to several factors including, although not limited to, the quality of items you will have as well as the actual cost of the cards….
Lets say you sell TOPPS Star Wars Master Collection Cases, they will set someone back over $2-2.5k or if you head down the PANINI basketball bouts you can almost double and triple that amount – all of which will also depend on the level you want to trade at?
This leads to stock investment, there are many dollars invested in holding stock.. sometimes holding for well over 2 – 4 years before profiting, if you intend to break and sell/trade…. So make sure money is ok 
Another obstacle is tackling the biggest online selling platform, where most buyers are bargain hunting.. Although, if you get a good card (rare) it can be ok as true buyers will pay over time… Another concern can be USD fluctuations, if selling online and if dealing with USA centric items… To combat this though, look at selling in USD through paypal 
– oops, just a heads up…
If selling Aussie cards and stuff, this is ok.. AFL and Cricket have a big following here with many options available and traded at all ages (read traded as buy and sell) and can be profitable.. One issue with Aussie items though, is that if you are not a shop it can be difficult to acquire a distributorship or even be able to buy wholesale as purely online sellers are still frowned upon by some – reason isnt what most expect, its generally not perceived as well as the seller is not considered overtly serious (joke) and it is also hard to draw the line between whether the seller is a seller or a hobbyist just trying to buy the cards at wholesale cost 
Ok, i have typed heaps… got a little distracted, sorry, although will wrap it up as had something pop up a sec ago in my email (WORK.. LOL)…. So, in short:
Awesome business if you LOVE it and can make some awesome customers although on the down side you will need a large cash play to start to ensure you have decent quantities to be considered a contender (or use your own stock on site/ehay ..lol)… There is lots of competition, although if researching products well and buying well you can be successful.
A quick note on ebay, you will also want to build up an account that has OUTSTANDING reviews to build confidence as lots of international key card sellers have accounts running 100% feedback with over 20,000 or more trades and some local Aussie players have 100% with over 2k trades…
Wishing you all the best and apologies for the length of post.
Jason
