(Apologies for stupid newbie investor questions... I read about this stuff in the news, but I have no idea how it actually works.)
The only problem is that if you invest only a few hundred bucks, trading commissions (~$20 to buy and sell) eat up a big portion of your (potential) returns. For example, if you invest $200 and come out of your trade even (selling bonds back to the market for $200), you still pay $20 in trading commissions, so you'd be already down 10% on your trade. In other words, you'd need to be up 10% on your trade (which is very nice) in order to just compensate for commissions.
If you want to help prop up the Greek govt, you probably have to give it money, that is, buy new bonds from it. It's probably not offering to pay 74% on such loans. And, given the yield of current bonds, it's probably not even bothering to try to sell bonds.
Well in some sense... if you create demand for the existing bonds then you create confidence in them, and hence drive down yields on the next round of bonds that get issued.