I have a guy willing to sell me some ammo and maybe preban magazines in California.  Is this legal?  Can I buy preban magazines in CA?

 Mags larger than 10 rounds can not be bought or sold in CA. Is this what you mean by preban?

 Also, you can't convert an SKS to a detachable magazine. And you can't put a fixed magazine with a capacity greater than 10 rounds.

 If you are worried about legality, I'd steer clear of any magazine swaps on the SKS.

 The ammo though is a different story. If you can get it for 0.10/round or less, buy it all. It used to be sold for that price everywhere, but now people have raised the prices to 0.13-0.15/round.

As Zapp states, no sales or transfers at all on high cap mags within Kalibanistan.