Lighting and camera stuff was already brought up, so I imagine you already have that in mind, but let me reiterate anyway; there was an incredible sounding sample clip for a video once, with pictures of a lee that looked hot, and the sound of her laughter was beautiful but... the people recording did a shit job. It was very dark, and appeared so that you could only see a circle in the middle... with the 'ler in the camera's way. Yeah, needless to say, I just deleted the sample immediately and didn't even look.
I don't think you/your girlfriend really understand what you're doing, though.
Yes, Camera/lighting/setting are all important, but I mean... editing is also important, too. And it's always a turn off to see some guy tickling a girl while he has a massive hard on, which I would suspect will happen in this setting. Plus, how experienced of a tickler are you? I don't want to see you get in the way of the camera, so that I can't see what's going on =/ I'd be disappointed. Worse, I imagine you won't have a camera-person with you, so that the point of view would become stale.
Beyond that though, you don't have a means to deliver the product or receive payment, and I'm not sure that I would just paypal you some money and hope for the best (though, probably with people giving reviews/etc. of the clip, I'd be more at ease to do so). I think that there is a lot you're not considering, and that you therefore should have her perform as a model instead of doing the clip yourselves. Talk to Tickle Abuse or something, I'm sure he could hook you up.
Also, sample clips; as I mentioned, I dunno if editing will be an issue for you? But I certainly won't be a faith buyer, as interested as I am; I'm going to need some sort of demonstration of the video so that I can ascertain whether or not it's worth the money. Pricing is another thing, too; most Clips for Sale do something along the lines of a dollar a minute, but have you considered price?
Anyway, regardless of my criticism, I'm interested; I just want to make sure you know what you're doing before you embark on this quest. I don't imagine that it will be nearly enough to help pay with the bills, but since this is already something you do, it certainly can't hurt. I plan on filming my future sessions for my own benefit, but I mean, if I could make a little money on the side and my lee was cool with it? Why not, you know?
One little thing, though; could you put up a picture or two of her, prefererbly barefoot so that we can see her soles? Just curious now.
At any rate, hit me up in PMs with your progress/if you're doing it/when the thing is up. I probably won't see the topics, and assuming that the product is just, you know, decent quality, I'll probably snag it to help you out.
Oh, and if you do go through with this, make sure to lube her feet up and give her a real good tickling; if she's not begging for you to stop and you don't keep going... hm. Well, it's a perfect opportunity to do it, and I would be sad that a girl wasn't pushed past her limits, I guess!
Etc~