As previously mentioned, all cities are selling fast. Within a couple of days, it seems most properties already have offers. This is due to the low inventory throughout Southern California. If you are ready to make a move now is the time to do it! Prices will be increasing steadily because of the demand for homes.
As for the zip codes that have the most cash transactions, I find in Lake Elsinore and Corona there are an increasing amount on a daily basis.
I hope you found this helpful, should you need any further assistance, please feel free to contact me.