What's it with eBay? You just can't get away from them. Their bulk "dictionary purchase" of ad terms from the Google Adwords/Adsense system means you find ads for eBay no matter what you're looking for, even if it's for toenail clippings or dead pets. Whatever the reason for their success you can join the bandwagon and sell your rubbish on eBay and convert a garage of old junk to hard cash.
And when you've acquired a fair amount of experience with selling your own rubbish, start selling others' rubbish. How to get your hands on enough trash? Place an ad in your local paper to do house clearances, or just buy stock from wholesalers in - yes, you guessed it - eBay itself. You can get anything from unused toasters to umbilical cord containers. A lot of ebay sellers are actually shopkeepers; they buy stock new from wholesalers and sell them in eBay instead of a physical bricks and mortar shop... and make a healthy living doing so.
You've now got a lot of experience buying and selling in eBay. It's time to write a book or ebook on how to use ebay to generate millions. It doesn't matter that you haven't made millions yourself. People still buy these books even if it's just to learn their way about ebay which can be quite daunting for some. And you can sell it on ... you guessed!
Don't fancy writing? Then set yourself up as an eBay assistant. eBay itself will send you people who need help with their complicated system. And you can charge them a commission.
Alternatively, setup a shop to accept goods from those who can't list them themselves. Yes, a physical shop. There are lots of them springing up all in big cities. The operation is simple. You take something in from somebody, sell it on ebay and retain a percentage for all your efforts.
Think building an ebay business is too much like building a normal business? If you've got the capital you can take the easy route. A lot of ebay "shop" (businesses) come up for sale in site-for-sale forums and with business brokers. The ebay username and feedback generally accompanies the business so you continue to benefit from accumulated goodwill (though the "transfer" is something that eBay may not approve of).
Opportunities to make money in eBay are limited only by your imagination. There's money in everything from making connections (putting people in touch with other people) to coming up with or commissioning software to make the average ebayer's life simpler or more productive (thousands of such programs already exist). There's even a business model involving just searching for and finding items that have been misspelt in the listings.
It's not just eBay. Any merchant big enough or third party affiliate manager program - from Commission Junction to Clickbank - has opportunities. For example, you can create an Amazon affiliate site.