Right, the obvious question is why does Owen buy 3PO in Ep. IV.

Well, for one thing several years have passed. 3PO is painted a different color (and there are other protocol droids that look like 3PO).

The other thing is that in SOTDS he turns to Beru and says, "Doesn't even speak Bocce." He's a protocol droid, and he can't even do that. He's good for nothing.

So if you look in Ep. IV in that light, Owen seems very wary of 3PO, and the resemblance is bothering him.

"Do you speak Bocce?" he says finally.

And of course 3PO claims that he does. So Owen buys him. The funny part being that he bought 3PO twice.

I still haven't figured out the memory wipe issue in Ep. III though...