If a person is looking to start a serious Buffy the Vampire Slayer Collection, here are a few things that are definites and awesome to collect:

1. Prop from the show - try to at least obtain one prop from the Fox studios
2. Prop replicas - there are some neat ones out there, such as the Sword of Angelus
3. Action figures - there are 3 major types on the market right now
1. 6" figures (ranging from the early Moore Action Collectibles to the Diamond Select)
2. 12" figures (Sideshow products that have some creepy likenesses to the characters)
3. Palz figures (these are like Lego people...and are actually popular)
4. Signed stake prop replica from favorite cast member (for example, my fav is Willow, so I have an Alyson Hannigan on display)
5. Convention exclusives (these are items that are only available at the conventions)
6. Trading cards (basic sets are fun to reminisce about good times but the coolest part of these are the pieceworks and autograph cards - fun to collect)

These are just some basics. This will help you get started, at least. If you want more in depth collecting, feel free to message me. I'm running a 'kinda' store right now called Willow's Wily Charms, devoted to the collecting of Buffy and Angel official products. =)