Is there a brand name on it? If so, she can call around and see what new ones are going for (after figuring out if it is, indeed, a 60 footer). If it’s in really good shape she might start the price at full price, or 10% off full price to start with–that way she has room to dicker, if she is so inclined. Frankly, if it’s not too rusted I would think it would pretty much hold its value. 
I would put ads up at local feed stores and tack shops and in horse publications (The Equine Marketer) or other papers that have “Livestock” sections. I, personally, would never list anything on Craigslist–too much bad news connected with CL. People getting stiffed for the cost of merchandise (robbed), or “stiffed” (as in murdered). It just isn’t worth the risk! :no: