Now.. I know that images for web viewing the PPI or DPI meas nothing, but If I'm selling a photo and the "person" buying wants to print and frame.. then wouldn't the PPI I saved the image at matter?
No, it won't matter. What matters is that the resolution is high enough that the customer can print it at the appropriate size. For example, a 6MP image is 3000px X 2000px. Regardless of whether the metadata tag says 72 or 300, that resolution will not change.
If I save an image at 72PPI, wont the image tell the printer to print at 72DPI? In this case wouldn't the image be bad in quality instead of saving the image at 300dpi?
Not with modern printers. Historically, you used to have to tell your input hardware (a scanner) how many lines to scan (both horizontally, and vertically to give you 'dots') per inch to determine the size and detail of the digital file you created. You then had to tell your printer how many dots per inch to output - by making the input and output values different you could increase or reduce the size of the output image. But this was in the days of professional scanner operators and printers.
These days, where printers are used by everyone (not just professionals), software does all the necessary math for you. If you tell the computer what size you want the output image, it will work out the necessary DPI to match the resolution of the file to the desired print size.
The DPI metadata tag found in digital photographs is pretty much irrelevant these days, and unfortunately, it causes a lot of confusion for photographers. The only time the DPI metadata tag will make any difference is when you open it in a programme such as Adobe InDesign because it will place the image on a page which has it's own DPI setting. If you're just printing it and not using it in some kind of publication design software then it won't matter.
If I want to send an image to print at ProDPI for example they request images at 300PPI, but a person buying my images might not know about this and If I saved in 72PPI that person might not know what to do..
Hopefully, your print supplier is not actually referring to the metadata tag. If they say they need images at 300DPI then that *should* just mean that they need the resolution of the image to be 300pixels for every inch you want to print. So if you want a 12x8 print, the resolution of your digital file would need to be at least 3600px X 2400px (and the tag won't matter at all).
However, if they are seriously telling you that they cannot print a file because the tag says 72dpi, but that it will print perfectly simply by changing the tag (without changing the image's actual resolution) then they do not have a clue what they're doing and it's time to find yourself a new print supplier.
At the end of the day, 300DPI is largely a myth. The only thing that matters is that your image has enough resolution that when you print it, you do not see individual pixels. The larger the print, the lower the DPI you need to use, because the viewer will get closer to a small print than they will a large one. For example, huge billboards obviously aren't printed at 300DPI - most probably aren't even printed at 1DPI.
Having said all that, there is absolutely no harm in changing the metadata tag in your image to read 300DPI. It makes no difference to the image, but might help keep people happy.
Alternatively, seriously consider using a service such as SmugMug to sell physical prints instead of digital files. That way the customer will get exactly the print they want delivered to their door without having to worry about resolution / DPI / etc. Other companies will offer a similar service.
Sorry for such a long response, but I really hope this information helps you understand the difference between the DPI tag and actual DPI.
Best of luck with your business venture!
Iain
