GST question on sale of horse - Help!

Hi there, this is my first forum and I am really hoping someone out there may be able to help me with a GST question. I have agreed to purchase a horse that was advertised on Horsetopia the price was listed as $3000. The advertisement did not say $3000 + GST nor did the seller mention GST when I went to see the horse. Now that I have agreed to buy the horse, the owner is now billing me for GST. My question is whether or not I am now legally responsible to pay the owner GST? Should I have assumed that GST would be added to the price? Frustrated and not finding any answers doing a web search.