I actually have a huge file folder filled with the old drawings, and every time one of my "working" folders gets too full (once or twice a year), I go back to the big file and toss out some old drawings to make room for newer ones. They are all arranged chronologically (new stuff goes in the front, basically) so the very oldest stuff is always the first to go.
My system kind of accounts for this automatically, since everything is archived by time: as in, by the time an old sketch finally ends up in the trash, it's already obsolete.
I mean, I'm not rescanning anything after it's been squirreled away for two years...might as well just re-draw it, 'cause it's not gonna be up to my current standards anyway...
Unless you're a pro working on a comic that's actually making money for you in the present day, I wouldn't worry about people wanting to buy your original sketches. Just being realistic. =/
And if you're really concerned about the off-chance of your comic exploding in popularity in the distant future, you could just save a few of the best pages: y'know, the ones you might like to keep as mementos regardless.
If someone just really wants to see your original sketches (for some reason...), I recommend keeping in mind that it's none of their damn business. ^^;
I mean, show them off if you still have them, but if you don't, it shouldn't be even a remotely big deal to you or to the person asking. Certainly not big enough for you to prioritize saving old pages that you wouldn't keep around otherwise.