so do i give them the serial number or no? just tell them i built a trailer? ant wa-la?
?? 
If theres a problem take it (call ahead ) to DMV explain to them the facts (eg "I bought this but didn't get titile ) , they will in turn have you fill out an afidavid and you must swear under penalty of purgory (a crime) that your stateing true facts , if a problem they will refer you to CHP (not likely) if alls goes they will assign a new vin and possibly a new plate. its a simple process assuming its not stolen or has fees due) . I have heard stories for years about folks "creating" a trailer or car , boat etc ?? simply filing the forms and dopey old DMV just stepping up. The reason they are doing so is because the individual perpetrating this transaction is signing a legal document saying what there are doing is legit the rub comes someday/maybe when someone lays claim (and proof) that it was thiers in some parking lot somewhere.

I'm a licensed CA dealer and deal with DMV regularly, call if I can help Tom
FWIW the serial # is the vin on older trailers