I have personally purchased many vehicles and trailers long distance without personally seeing the items. I have always had a friend or a "Hired" inspector/appraiser physically look at the item. There are many ways to confirm if the seller is real.
Google is your friend.
Ask for the address. Use google earth to see if the trailer might be sitting a their location in the Google earth pictures. Is there a real location at that address. Get a copy of the
title. Call the state DMV to confirm that the
title number is the current title. Is the title in the sellers name or are they selling "Open Title" which by the way is illegal in every state in the union. Is the person you are talking to real? What's their name? Needless to say the internet is your friend and can "help" determine if the seller is real. It certainly will not confirm it but it "can" help.
Ask yourself this question:
If you are UNWILLING OR UNABLE to go look at trailer or anything else for that matter that is not local to you why in the world are you shopping for a trailer a "long distance away" from your home?
There are many answers to this question ALL of which make sense to the buyer when they want something. So this begs a second question:
Can YOU afford to LOSE YOUR MONEY?
If not as suggested above HIRE someone local to complete a pre-purchase inspection. Have that inspector go to the sellers location to inspect the trailer on your behalf and present a detailed report on condition and does it exist. This way you know where the trailer is really located and not vacant lot for an address. This will be the BEST $100-$250 you will ever spend in your hunt for a trailer.
My experience over the last 30+ years has confirmed that for every $10K I am willing to spend on a vehicle not local I budget $1K for travel to go look at vehicles. $20K vehicle $2k travel budget. This "travel" budget let's me look at more vehicles than are available locally and I ALWAYS find the one poor SOB with a much nicer vehicle than all the others that really needs to sell and that means a MUCH better deal that far and away makes up for the expense of travel. If I do not have a friend or a "Paid Liar" (Local Appraiser) available locally to personally inspect the long distance item I am interested in I pass on that item.
The "feeling" that if I don't act quickly this trailer will get
sold before I get there is a strong emotion and seller's know it.
CL is the worst with the invention of national CL search engines. People from all over the country call on items not local to them listed on CL. When selling I list items on CL that I am not willing to ship. Items I am willing to ship go on Ebay or other national selling platform. Everyone wants to negotiate on the telephone on something they have never seen. How dumb is that??? Come look and we can make a deal.
Long winded answer with some personal opinions that make little difference however you did ask.
Good luck with your purchase.
You "May" need it!!!