Their reported fraud rate is 1/10,000. There is reason to believe that this figure is somewhat low. Other shopping channels have a shrinkage of about 1-2%. I personally doubt that eBay's is that high. (People are generally more willing to steal from big companies than other individuals...)
Let's say that their actual fraud rate is 1%. Suppose that you get a 10% better deal from eBay than you do elsewhere. (Odds are that that's slightly pessimistic as well...)
After 10 times you've probably not been defrauded, and if you have then you've almost certainly not been defrauded twice. (If a 1% fraud rate, the odds that it has happened once are about 10%, and twice are below 0.2%.) Even if you've been defrauded once, you're still even. If you haven't been defrauded, you're ahead.
This is the same kind of numbers game that is used as the basis of insurance. But on eBay the numbers are favourable enough that it takes very little time to get to the point where it is worthwhile.
This is before taking into account the things that eBay does to try to make buying through them a more pleasant experience. You can also choose to minimize your risk in various ways, for instance you can only buy from eBay sellers who've done a lot of selling. (Lots of people run businesses there, and so you get a *lot* of feedback on them.)
If you don't want to use eBay, that is your choice. But not doing it because you're scared of being robbed is being penny wise and pound foolish. The risk really is worth it.
Cheers,
Ben
PS Disclaimer. I work for eBay, though in a capacity that has nothing to do with their auction site.