It really is 2 different things. an online auction and a live auction night of and most places will also run a silent auction. Silent and online are similar in "stuff" quality and a live auction is for the "special" high value items (good sports tikets, great vacations, principal for the day etc)
i use a program called MaestrSoft/ MaestroWeb this program can do all of the stucc Cmarket can't like keeping track of all of your donations/descriptions/cash donations/rsvp's, live silent and online auctions and then downloads into the software program which you use to print all of the gift certificates and bid sheeta log with donation thank you letters.
When you have an online auction it really is best to finish it online. The acception is items that do not end up having a bid on them can be moved to the silent auction for night of the event. if you start items online and move them inside you agravate buyers. People who can't cone don't bid because they can't be there to finish and night of bidders get annyed because the starting bid is too hight to "get the party Started"
no more than 20 items for your live auction 15 is really the ideal number, you won't loose people, keep the items general, it is a bad place for class gifts, there are not enough people iterested in any one item to keep the bidding going.
The key to any auction, online, silent, live is to be creative. don't just hit up local businesses for GC, find crafts, palnts, vacation homes, tickets to events, catered parties etc. look at other auctions online and look to see what other schools like you are finding. ie if you are and enlementary school a trip to the fire department birthday party with a cake thrown in can be a lot of fun. ice time can be a great idea for a party, local hotels will donate an hour of pool time, pair it with pizza and grea get together....
As far as an auctioneer, it depends, sometimes a funny teacher/parent at school can do as good a job and will donate their time. Sometimes a local personality, tv or radio can be fune, but you will pay unless they are a member of your school.
Getting started over the summer is a good idea, get the paliminary done of picking a program that will suit your needs as a school, there are some good programs out there, take the time to evaluate where your money is best spent. Cmarket for example says they are free, yet on closer look on a sliding scale they take 10% of the money yu bring in. think about what you want to sell and how you are going to go about getting it.
good luck!
-DC