Over the years we've purchased several out of state trailers and vehicles. Without clear titles and bills of sale in hand, we could not have registered them in PA. Most of our transactions were handled through our local AAA office, who gave us a heads up on exactly what paper work was needed, and at times inspected the vehicles for the proper ID numbers. Others were inspected by the local state police and repair facilities that offered state inspections.

Talk to anyone who actually handles the title transfers in state for advice. We've purchased both light weight and heavy weight trailers, from single axles with no brakes (under 2990#) and multiple axles with electric brakes. The paperwork on each was comparable, and without the titles, the transfers could not have been effected. 

There is a waver for trailers considered "home built" which allows an individual to "build" his own. This involves a whole other set of regulations and paperwork, and the assignment of a state ID number. It is not an easy process.

Best advise is to not purchase anything that you're going to bring in state, that doesn't have a clear title, that is both signed and notarised by the seller in the state in which it was purchased. If the seller cannot provide a clear title at the time of the sale, walk away. Have him take you to a notary, to effect the title transfer before you leave with the trailer. Do not give him any money, until the notary hands you a properly transfered title that is acceptable in PA.

Make sure that it is the seller's name on the title. Some owners will not record their transactions and names in an effort to aviod registration fees. A title from anyone other than the last seller, will invalidate your registration process.