Each situation will be different so what might be good in one situation may not apply in another.
If the trailer was on ebay and the seller's feedback is not only longstanding but also current, paypal is the way to go because you are usually covered. By current I mean the seller has been using the account on a regular basis. An account that shows no activity for a long period of time suddenly listing a high ticket item wanting to handle all the finances outside of paypal raises suspicion.
Does the sellers phone # match the address where the trailer is? Does that match where they want funds sent to? Is there someone you know or even someone from here that can verify a trailer exists? I know up here, postal money orders over $50 or so require ID to cash which must match the info on the money order.
Keep your deposits small, and follow the cautious side of your intuition. If the seller is demanding something you are not comfortable with either negotiate terms that you are comfortable with or back out because there will always be another trailer.
There are 2 threads in the sticky posts in the referrals forum
that discuss many things to be cautious of.