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If you like saving money, buying HP inkjet cartridges on Ebay is the way to go, as long as you only buy ones that clearly state the expiration dates in the listing!  If they don't, avoid buying them!  I see Sellers offering OLD ones (circa 2004-2009) and pretending they'll work fine. DON'T BELIEVE IT.

I have a 2002 Model HP v40 inkjet printer, which has been a workhorse printer for 10 years.  Along the way, I've learned some great tricks for problems when changing the printer ink cartridges. 
NOTE:  I do not work for HP, I'm not a printer tech----this advice is my own, gained from actual experience for over 10 years, and may (or may not) work for your particular printer situation.

General Advice:
1.  Don't buy re-manufactured HP 78 cartridges.  The ads say they work, they may even offer a "guarantee," but they generally DON'T WORK.  I've tried refilling them myself, and buying re-manufactured ones....My HP v40 is a sensitive printer, and will not accept re-manufactured cartridges.  You end up wasting time and money.
2. Watch those package expiration dates!  My experience is that the BLACK refills will generally work up to 3 yrs past box expiration dates, if they are still in a factory sealed box (with green pull tape).
Tri-color HP 78 cartridges are more prone to drying out, so best to use ones no more than 2 years past expiration. It's better to keep shopping and pay a bit more for a newer one, than get a "bargain" cartridge that doesn't work!
When you get color printing problems, before you give up and buy new cartridges, use the HP Director Tool (in your installed software), and run "Clean Print Heads."  Sometimes that's all you need to get your colored ink flowing!

If you get a HP 78 Tricolor that installs and aligns fine, but you get error messages ("Remove and Check Color Cartridge"), or you print a page that seems faint, dull, or missing one of the 3 main colors (red, blue & yellow), here are 2 great tricks to try:

For the "Remove & Check Cartridge" error message:
a) Open your printer cartridge door and let the cartridges come under the door to remove. Then UNPLUG PRINTER.  Lift the door and remove each cartridge, set them on their sides, on a clean paper towel.
b) Moisten some Q-tip swabs with regular isopropyl alcohol, not distilled water (as HP suggests). Get a flashlight and peek inside printer, to gently wipe the silver contacts contacts up and down, to remove any debris or ink.  Don't leave any fuzzies behind!  Do one side at a time, may take 3-4 swabs until no more ink comes off.  Let contacts dry 5 minutes.

c) Meanwhile, take the bad cartridge, and gently wipe the little rows of copper contact dots along the edge (NOT THE BIG SQUARE COPPER CONTACT ON BOTTOM OF CARTRIDGE) with an alcohol moistened swab, until no dirt is seen on swab.  Let dry on paper towel 5 minutes.

d) Re-install each cartridge, snap the latches on them and plug the power in last.  The "Align Page" prompt should show on the screen, so do the normal Align procedure.  If the error message still shows, CLEAN one more time.  If a second cleaning doesn't work, the cartridge is no good.

(For Tri-Color #78 Cartridges Only Printing One Color)
I've bought them a year or two past expiration date, and sometimes they print fine for maybe 10 sheets, then only one color would print...or they suddenly start missing printing one color (yellow, blue or magenta).  I've found the method to "revive" these problematic HP 78 Tricolor cartridges:

a) Run about an inch of hot tap water (not boiling, but hot) in a small bowl.  Have a clean, folded paper towel ready.  Carefully DIP the cartridge (copper end down) into water 3-5 seconds MAX, then GENTLY touch the square copper bottom (the print head) end on the paper towel a few seconds. You'll see what colors you have working...should be 3 lines --- red (magenta), blue and yellow. 
b) If ONE OF THE 3 colors is missing, here's the trick--- and don't think I'm crazy---holding your cartridge upright over the paper towel, look for 3 very small round "holes/indents" on the TOP of the HP 78 cartridge. They appear to be totally sealed, but they aren't.  Lean over the towel, holding the cartridge UPRIGHT, purse your lips against the little "holes," and blow hard a few seconds---A drop or two of colored ink will plop out of the bottom, onto the paper towel!  If nothing comes out from several good blows, re-dip in water and blow again.  Once you get a drop of each color you were missing, shake the cartridge good for 5 seconds, then carefully re-wipe the copper contact dots on the side of the cartridge with alcohol dipped Q-tip swab, let dry and re-install. (DON'T "SOAK" THE CARTRIDGE, OR YOU'LL RUIN IT).
c) Run a printer test page.  All 3 colors should show up!

This trick will generally work best on HP 78 Tricolor cartridges less than two years past expiration.
Don't expect miracles trying to revive older ones!
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