As someone who does own a horse with true early onset melanomas (he already had them at age 3) - and who was as stupid as someone could be when buying a horse with this problem - please pay attention to what folks are telling you above.
I am extremely fortunate that my horse has responded to Oncept (the dog vaccine) and I love him to pieces and have a good job, so the expense of keeping him clear of melanomas is worth it to me. Just FYI, he’s had 7 injections this year, which is approx. $2800, doled out in $400 increments. He’ll be down to 2 injections a year from here out, so that’s reasonable, to me. I also got a deal from OSU, I think because he was patient #1 on Oncept, and believe they are charging people more in the range of $500 per injection now.
I think it’s fair to say that the state of melanoma knowledge at this point is so sketchy that even vets in the know a) can’t tell you that the horse under consideration isn’t already riddled with them internally b) can’t tell you whether the vaccines available will work on any given horse and c) can’t predict at all what the projected spread of his melanomas will be. So getting a vet’s opinion will be just that - not many facts to help you make a buy/no buy decision.
You would be taking a huge risk. The fact that they want $17K for him is delusional IMHO. They are not only getting someone else to take this poor horse off their hands, they’re getting a ridiculous amount of money while taking advantage of you.
Use the search feature and read all the melanoma threads on COTH. If you decide to buy him, at least educate yourself about what you are getting into. And make them adjust the price accordingly.