Hi Scott! (Nice name, by the way!)
My two friends have answered your questions, I just want to endorse what they have said! Conversion pumps allow me to use whatever color ink I choose. And not just color, but brand as well. I have a nice collection of Private Reserve Ink (Tanzanite is my favorite!), but also have some Parker Quink and some Cross black as well. Using the different inks is part of the fun of using fountain pens!
And if you're serious about selling fountain pens to people who are serious about fountain pens, I recommend you sell your pens with a conversion pump from Schmidt, probably the best conversion pump available that fits the pens we make. They're made in Germany, and Berea usually keeps them available. Also, if you're selling fountain pens, I like Doghouse's suggestion of keeping a fountain pen you use so people can try it and see how the nib feels when broken in. I wouldn't let anybody ink a new fountain pen unless they have already bought it. Keep some good paper available for them to write on - bad paper can lose you a sale!
One of the best pieces of advice I ever got about selling fountain pens (thanks Russ!) is that to sell fountain pens, you should know how to use them. Buy a couple of nice fountain pens and use them as often as you can. A fountain pen fanatic can tell if you don't know the subject, and will shy away from you. If you want to invest in a good pen that has a gold nib, and also happens to be very cool, look into a Namiki Vanishing Point. Nice pen! But I also bought a Parker Sonnet with an 18K nib about a month ago, and it is turning out very nice as well!
I hope this helps!
Scott.