I would utilize a bonded dealer and pay them a service fee for handling the transaction. Any trailer or Car dealer should be  able to handle that. 

Or use a Escrow officer at a title company.  When the title is in their hands and properly signed over, they can wire the money into the sellers account.

Or fly to the sellers location and inspect the trailer, pickup the title and give them their money in person. Have your transport company standing by to haul the trailer off as soon as the transaction is complete.

Be careful of a recent scam.  Where bad people request a replacement title, go get a new loan that encumbers the property,  using the new replacement title as security. When you pick up the trailer, They give you the old title they claimed as being lost.  It doesn't show an encumberance.  When you try to transfer title, you learn from the state that you are second or behind the first lien holder. The first lein holder takes the trailer and you are left trying to find the seller.  Just because you have an official title issued by the state, Doesn't mean a second title hasn't been issued.  Check with the state and make sure the title is clear on their records.