Registering trailer in CA
I went through this in CA just a year or so ago. Not that bad. I bought a small trailer knowing that the seller had "lost" the pink slip. When I got home and checked the plate with the DMV, I was not very surprised when it turned out the plate came from a big travel trailer in Southern Cal! Clearly this trailer had never really been registered before.
The trailer had some manufactured parts, but no VIN anywhere, so it was assumed, I think, to be homemade. In any event, the DMV has a form for EVERY situation and actually handled this pretty smoothly.
I took the trailer to my local small town DMV office and it was pretty easy, though there was some confusion among the employees there. One told me I had to fill out some paperwork, then haul the trailer across town to the CHP office to be inspected, then bring it back to the DMV to finish the process. For some reason I now forget, I had a chance to ask the other employee in that same office. She said not necessary, dug out a little permanent new stick-on VIN number, went out the parking lot, stuck it on the trailer, then went in and finished up the paperwork in about 10 minutes, handed me a new plate, collected some money, and sent me on my way. Easy!
There is one thing to remember. They will make you pay "use tax" (same rate as local sales tax) on the purchase price. You might want to think hard about what to tell them you paid for that old, beat up small trailer (hint, hint).
Good luck. Hope your experience will be as easy as mine was.