Are they under any obligation to ground the value of their own stock or can a salesman simply claim that the "true" value of that stock is much much more than it currently seems to be?

Stock is worth exactly what people will pay for it. Ebay share holders get to vote to accept or reject this deal

Presumably stock market valuation is grounding?

Also, eBay shareholders can vote down the acquisition if they don't think the deal is good for them.