Hey Eskimo -
I applaud you for not only coming up with a few ideas, but for also having the balls it takes (around here) to ask for advice...
I think either of these ideas could work... you just need to tweek them a bit. A dating site for those under 30 is o.k., and it already exists in one form or another... but you want to advertise and sell to these potential daters, instead of set them up in another dating program where you are paid... If it is a great site with good & unique content for the viewers, then it has potential.
I actually thought the classified idea is not bad, but I would look the other way - Like how much do I need for each person to place their add (no matter the item cost maybe???) So, we list any item, any cost, for 30 days for $3 bucks type thing... You could also help people sell their own stuff from your classifieds by setting them up so they can list it (their sales items) on the MySpace, FaceBook or whatever... more traffic (more sellers?).
Plus maybe you are out on Gbay making sales of their items (promoting your classified site) as well, plus maybe pushing your other (affiliate) programs from there.
I say tweek it and get started... we all even tweek as we go sometimes, just grab an idea and start it knowing it can change direction with your research and analysis - so you already know it will morph - be ready for it as your eyes open to new ideas you hear.
I think for some online dating fun - There should be a dating site where you can set-up your friend with a date your friends found for you online ??? Could be fun too!
Totally, Like "The Set-up .com"