In CA transfering a firearm can cost close to $50.
I just posted all of the information a few days ago on a similar thread. Private party transfers can go through an FFL for not too much (plus other expenses like a background check etc).
However most cheap handguns are banned as "saturday night specials". Such firearms can only make it into CA with someone moving to CA and becoming a CA citizen from another state, and that person must pay a fee (currently $19 per handgun) for every handgun in order to register it and legaly possess it in the state of CA.
http://ag.ca.gov/firearms/forms/pdf/ab991frm.pdf
Store purchases require one to pay for both a background check, the record of sale etc. That usualy totals around $30.
There is also CA sales tax, and if it comes from out of state and is approved for sale in CA, shipping.
What all of that means is there is very few cheap handguns in CA that one could make much profit with these.
Possessing an unregistered "assault weapon" is already a significant crime itself. So I don't see people that went through the time and effort to register thiers turning them in.
That really just leaves handguns. So you need to find a handgun that both the purchase price, and the gas to travel to get it, and the ~$50 of expenses beyond just the sticker price leaves you with profit at a buyback of $250, and then ask yourself if there is profit if it is a worthwhile hourly wage divided by the amount of time it takes to accomplish (maybe an hour or two of overtime would be a better investment?)
There is also a one gun a month law in CA.