[Mark]

Many "off the shelf" trailers can be adjusted by the trailer dealer/manufacturer to fit different boats. However, some trailers are intended for deep-v hulls and some are intended for boats with other hull shapes.
If you are not really familiar with trailers, I would avoid buying a used one from some guy in the paper who assures you that it will fit your boat!

I'm not sure what SoCal trailers cost, but you can get single axle trailers on the east coast brand new for about $1400 that will be ideal for a SC18 - at least that is what I paid for a new Load-Rite last year.