When you have enough items (50-200) then organize and advertise the bumper. The organizing is pretty easy. Just buy yourself plenty of cloakroom tickets and get the donated items to the venue (with about 7-8 volunteers).
On the night of the bumper you sell tickets and raffle each item (or group of items). Its like a whole bunch of mini-raffles in one night. People love them and theyre great earners because you dont have any costs (apart from the cloakroom tickets, which are cheap).
Spread out your star items over the course of the evening and keep your absolute best items until near the end. This helps keep people there longer. The whole thing shouldnt last anymore than 2-3 hours since bumpers are popular with families with young children and they get bored easily.
One thing that helps bring people back to future bumpers is having a tea / coffee stall. Parents like to sit around and drink coffee while the children get a kick out of running up and down to buy the tickets. Incidentally the tickets should range between 50 cents and $2 depending on the item being raffled. Its one of the fundraising events that you'd run just once a year (maybe twice at a push).