You're right-- eBay is a great source of OEM inks, and neither I nor a pro photographer who owns an HP certified repair center I've talked to have ever had any problem with expired ink carts or printheads. I've used 11 year old printheads and they're fine. I buy inks from macroenter.com, usually. Sometimes they can't source every color, in which case I go to InkOwl.com which seem equally good, but more expensive. Macroenter's customer service is exceptional, and I don't have to purge new printheads since their inks are exactly the same color as OEM. I use a couple of gallons of ink annually, and my margin would be far narrower if I stuck to OEM inks.
The printers themselves operate exactly the same with aftermarket ink. The only unknowns are their acidity and longevity. All I can say is I haven't noticed any fading in prints I've had for eight or ten years.
I have considerable experience converting to 3rd party inks, so feel free to hit me up for advise should you decide to go that route. I made a ton of mistakes and spent far too much money figuring it out, but I now have a system that's worked flawlessly for several years.