I would say that it's going to come down to what your
state requires. That's because, for example, some states do not even title
(don't require and won't give) small trailers. So if you were buying such a trailer and lived in one of those states everything would be fine without one.
Each state seems to have their own rules.
Some states (NC, for example) allow you to title
"home built" trailers, and I looked at one Scamp
wherein the owner had titled it that way (he'd bought without a title).
On the other hand, if the seller is in a titling state, and you are in a titling state, that can make it more difficult. I have bought things from non-titling states and then titled them in my state without too much hassle (but, still had to file special papers, wait, etc.), but part of that process included proving that I bought it from a non-titling state. So if the trailer is now in a titling state, you might be more motivated to have the seller deal with it? (I realize that all sellers may not be willing to do this.)
If it were me, I would check with my DMV. Then know that when you go to actually do it, some good/bad/hassle/easy will still depend on who you actually end up working with when you get there (DMV).