I'm glad the owner will have the vet out soon; as you already know, there are alot of possibles.
My handy, dandy HO Veterinary Handbook says
"Nosebleeds do not ocur spontaneously in horses as they do in children.The majority are related to gutteral pouch mycosis, infections, tumors, and ethmoidal hematomas (more on this). Bleeding tends to occur intermittently, often with the horse at rest. The blood may be mixed with a mucopurulent discharge" (that even sounds yucky!)
"Bleeding from guttural pouch mycosis can be massive and life-threatening. Major bleeds are often preceded by intermittent minor bleeds from one nostril.
"Trauma to the face, especially when accompanied by a fracture of the nasal bones, is another cause of nosebleeds.
"A nosebleed may be a manifestation of a clotting disorder such as hemophilia, liver disease, warfarin poisoning, or dicumarol (moldy sweet clover) poisoning. These causes are rare.
Infrequently, blood will be seen at the nostrils - suggesting a nosebleed - but the actual site of bleeding is in the lungs.
"Progressive ethmoidal hematoma affects the nasal turbinates. This nonmalignant tumor is actually a growth of fibrous connective tissue and blood vessels. The tumor-mass can become quite large, fill the entire nasal cavity and eventually become visible at the nostril. The cause is unknown.
It goes on to say that malignant tumors are usually already metastisized when symptoms occur and treatment is usually "nonproductive".
Well, now that I've had my usual scare from reading the vet manual...
Keep us informed. It sounds like this poor guy has more than one thing going on from long term neglect and abuse. Was he supposed to be a rescue? With what you are describing overall, it sounds like the buyer should have been able to clearly see that there were major health issues. I hope he can be "fixed".
I am not one of those who in expressing opinions confines themselves to facts. - Mark Twain
The fact that man knows right from wrong proves his intellectual superiority to other creatures; but the fact that he can do wrong proves his moral inferiority to any creature that cannot. - Mark Twain