Look closely at the pics, I always look out for haircuts, pen marks, any other marks (hair colour can come off onto ponies, as can the colouring from saddles, bridles, ponywear, even newspapers or whatever the pony was wrapped up in) cancer, pindot mold, hair frizz, chew marks, and paint rubs. But it all depends on how good a condition you want your ponies to be in. I can sort out pindot, minor marks and frizzy hair, and I'm Ok with small hair trims and paint rubs, but I don't like my ponies to have huge non-removable marks like pen, cancer etc. But that's just me on my collection, everyone is different.
If in any doubt ask the seller. A good seller won't mind a couple of questions and will be happy to help you. Remember though that some sellers aren't collecters and won't have a clue how long the ponies manes or tails are supposed to be, and won't know what pindot is or that pen marks are a bad thing!