And your question is . . . ?
It depends on the area of the country, but in many areas you'll find more trailers sold privately than listed on the local MLS. So, if you're looking, certainly work with a Realtor but also recognize that many will be for sale that aren't on the MLS.
Financing can be difficult unless it's new or nearly new. There are some specialty lenders for mobile (and manufactured) homes. Usually, the park management will have a list of possible lenders. Many mobile and manufactured homes, though, are bought with either all cash or with owner financing.
Hope that helps.