Ebay is safe if you follow basic precautions... check their feedback, make sure they have a 100+ (1000+ to be safe) and make sure they have sold power in the past (if possible). This usually means you pay a premium, but at least you get power. You can also check out
www.magictraders.com and trade with anyone with over 100 refs, or trade through a 3rd party (use a moderator).
Lots of people on this site have extra power, as do tournaments if you can travel to one.
PM me for more discussion.