Could someone help me make sure I can use these?

[ExpAnonColin]

They're LEDs.

LUMINOUS INTENSITY-MCD:3000 TYPICAL/4000MAX
REVERSE VOLTAGE:5.0 V
DC FORWARD VOLTAGE:1.7V TYPICAL
DC FORWARD CURRENT:20~30mA

The only reason I ask is because the guy selling them says "you will get 100 resistors for working at 12V."  Is he just assuming that everyone works off of 12V?

-Colin

[Mike Burgundy]

LEDs want a certain current to work well. They're just diodes, but they emit light(hence the name Light emitting Diodes). The current through them, when you've passed their threshold voltage, determines the brightness. Do NOT go beyond the stated working current- aside from that, just make sure they get enough current to light, but not too much. Usually high-brightness LEDs will be used under their max rating - tune to taste - just make sure you don't exceed max ratings.
If a LED wants 20mA (max), start with a resistor that allows that current (this ignores the diode voltage drop and resistance, so it's on the safe side) or higher and see from there. You'll usually run the LED as low as you can (visibility is the issue here) to keep the battery as happy as can be for as long as possible.
A 9V supply with a 20mA will lead to a 450 Ohm resistor (V=I*R): use a 470. At least. This will insure not blowing the LED but will possibly (for high-efficiency LEDs) raise battery current unnecessarily - these LEDs can be quite visible enough with much, much lower currents.
When in doubt, stick in a 1k and see what happens.

[ExpAnonColin]

Thanks for doing the math for me!  Just wanted to make sure I could use a 9v since he said 12v in the description.

-Colin