I can see both sides in this issue.
My opinion is:
Many people do not want to sell their Horse, but may be forced to, so calls or email, etc. may keep bringing up the wound of having to sell a friend.
Many people that sell a Horse are in a business, similar to selling a car, once the sale is done it is done.
As with any thing I buy, I do the research and ask questions before the purchase, not saying that with a Horse there may be something unexpected I may need to know, but I would not contact the seller more than once, unless the seller offered, even then I probably would not contact them more than once afterward, not saying how anyone should do it, just my preference.
I bought a Horse once and the seller called me every month asking how the Horse was, was it eating OK, etc, etc. It really got on my nerves after 6 or 7 months, I am glad she finally gave up, I never try to be rude, but I thought I might have to be if she didn't quit.