My previous experience is that if the trailer (or other type vehicle) seller does not have a title in their name it is a great idea to walk away. (Does not include buying from a dealer, only private sellers. ) The registration does not have to be current, but the title must be clear.
My brother took her my dad's 1980 Shasta for a fish camp trailer three years ago. Dad had not kept the registration current since 1995. Cost my brother 12 years of registration fees before the title could be trf to his name.
Yes, it may be possible to obtain a title after a search. But you have an equal chance of finding the trailer has a repair shop lein, is stolen, has been 'totaled' due to water damage, etc.
Hear stories about such problems all the time about purchases from Craig's list.
Beware. Be very cautious before you give anyone money.