I agree with Cori.
I've been in a similar situation where I had no control over when I was getting my horse and the person bringing her kept monkeying around (that's what it felt like to me, the shipper probably didn't intend that at all) with the date she was coming and I was getting SO stressed.
I was really worried that something had happened to Lacey or that they weren't actually going to bring her to me, that they were just going to take my money and be like "seeya!" since I had no paperwork at that time saying Lacey was mine.
I ended up calling them like everyday and FINALLY after three weeks of monkeying around, they brought her to me.
I'm not saying you're doing that (I know you aren't) but the buyer doesn't know you personally and she has no previous knowledge of you to base her expectations on. For all she knows, you could be taking her money and running. There are lots of creeps out there and she's obviously just stressed about that.
I'd let her know your feelings and set a date that he will be in her possession by, no matter what, so that she can feel listened to and reassured that she will get her horse.
Good luck! Buying a horse is stressful for both parties. :)
Fabio - 13 year old Arabian/Lipizzaner gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.