We all know that printer manufacturers make most of their money on ink rather than printers - in the same way that petrol stations rely on their convenience stores for profiits rather than just petrol itself. So if there is a lot of profit in ink and other consumables then shopping around will save you money as you find manufacturers and suppliers who make less profit to offer you a better value product.
This is where compatible ink cartridges come in. If official manufacturers such as HP and Epson want £30 for a colour cartridge, third party manufacturers will sort you out from less than a fiver. Sometimes quality is an issue - occasionally a compatible ink cartridge will not agree with a specific printer, but most manufacturers of compatible cartridges are reputable and have reasonable levels of quality control, making them a legitimate alternative to official OEM cartridges. Despite official attempts to suggest otherwise, using compatible cartridges cannot affect the warranty on your printer - providing you do not modify or tamper with the printer itself then you are free to use any brand of ink.
So now on to Ebay. Ebays global market place is ideal suppliers of compatible and remanufactured ink products because the low cost of advertising means no massive costs get passed on to you, the customer. Also, competition between regular ebay-ers means that offers such as free postage or discounted multi-packs are common.
And now a word of caution. Just because it is on ebay doesn't mean the ink you can see is the best bargain. There can only ever be one cheapest source of anything, and it isn't necessarily always on this fine auction site. Visit a site like inkcomparison.co.uk before you commit on ebay - that way you can see what you would save over other online retailers - I use a lot of ink and purchase a lot from ebay but without a reference I would sometimes end up paying over the odds. If a search outside of ebay yields a price within 5-10% of the ebay price then I'll stick to ebay - it's good to support ebay and it's a nice vehicle for building up ebay ratings, but if the price is significantly better then why bite off your nose to spite your face?