I'd say that you need to speak to the OWNER of the horse over the phone by a certain date or you are no longer interested. If they can't offer that courtesy, then you pretty much have your answer. If they are genuine sellers looking to find a home for their horse, then that should light a fire under their butts.
I understand both sides of the issue. In my search for a horse I looked at a few different horses a few hours away on two occasions. On the first trip we went to look at a mare that sounded like she fit the description, but almost no other info about her was provided. I rode her twice and she was wonderful, but declined to buy her for reasons unrelated to the sellers (liked another horse better, but he didn't pass the vet). Turned out the horse was owned by a cowboy that knew nothing about English horses, and he had no clue how to sell them to a non-local if that makes sense. I probably wouldn't have looked at the horse were we not going to be in the area anyway because of the cruddy ad, but I'm glad that we gave her a shot.
The second time we went in the same general area we only went to try out one mare that seemed perfect for me. The seller was very accommodating, easy to get along with, etc. All interactions with her were pleasant, and we had two lovely rides on the mare. However, there were a few little flags that went off that made both my trainer and me a bit uneasy. A couple of things that the seller said such as how she had bred the mare several years back, and how she didn't become much calmer like other mares after being bred. Little comments like that which probably wouldn't have slipped out if we hadn't spent lots of time around her.
We ended up taking that mare on trial, then became aware of some of her "issues" (spooky and prone to bucking) from a concerned third party. We sent her back immediately.
If I hadn't gone to see that first mare despite the cruddy sales ad and lack of communication, I could have passed on a wonderful horse. However, if I had listened to my gut on the second mare I wouldn't have come dangerously close to buying a dangerous horse!! If nothing else, I would try to find any other horses in your prospective horse's general area and make a trip of it. This way you won't have made the trip for nothing, you'll be exposing yourself to more horse options, and you won't have blinders on because you're so desperately wanting to like this one particular horse.