Dont see how this would be worth the time compared to dallies. Only takes a few hours to do 25 on 5 characters and that's about 1300g. Even if you're lazy, the grizzly hills dallies yield 300g in literally about 5 minutes. Then there's a pretty easy round of Icecrown dallies that will yield another 600g in about 20 minutes.

I think you'd spend more time than that just spamming trade, let alone getting to the instance and doing the run. If you could get them to pay for several runs and be able to clear them in a few minutes, well, maybe it'd be worth it. But still... just hard to beat dallies.

Plus - consider this. If you are letting them keep the loot, you could run your own instances and keep the loot or do a heroic. Given the abyss crystals and the frozen orb, everything else, you're making more than 200g anyway without having to wait for someone to agree to pay you.

I've tried this with twink runs and other low level instances, and people won't pay more than like 20g, because some other bored 80 will do it for less anyway, without even doing the math on how little they're making. I've tried to fill full groups, charge them 20g each, then they complain that I'm making 80g like it's some kind of crime, and some just want the loot so they want a solo run. I just haven't found a way to make this worthwhile.