If you say it starts at 8am, there will be people there from 7am knocking on your door even if your curtains are closed and all the lights are off.
Garage Sale-ers are pikey bargain hunting gypsy scavenging freaks, and/or people who own/run second hand clap trap junk stores or make their living on Trade Me...anything with a price, they'll want to pay half that price even if it's something they've wanted for years and is a bargain at the price it's listed at
DVDs, you're dreaming if you think anyone will pay $5 each disc, you'll sell maybe 5%-10% of them like that, the rest won't move other than someone saying "I'll give you X for the whole lot" (X being variable in relation to how many disc there is but more likely in the region of $2 a disc)
It is a good way to clear your crap and you will get money for it, but not as much as taking the time to do it on Tard Me, but I'm with you that that is a pain in the ass, on going lenghty hassle with getting money and posting stuff away whereas this will all be over and done with by 11am...though you'll still have a load of crap no one wanted