Amazon has a lot of options available, and as long as you're getting one with good reviews and not at bottom-of-the-barrel prices, you should do all right
Hobbyist magazines, already tracking prices of sports trading cards, engaged with the Magic secondary market by surveying the stores to inquire on current prices to cards, which they then published
Undying (When this creature dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.) Use the power of the Wraiths as the triggered ability resolves to determine the value of X
See also Elderfang Disciple that's 35 cents, and Nezumi Informant and Virus Beetle
Rally received an offer for a buy-out on the set of $600,000 and initiated a poll earlier this week, only for another Rally user to counter the offer with a higher bid