Interest in High Standard Sentinels has been going up, and that one is nickel plated, which is a scarce finish for them, and in the box, which collectors always like. Still, I think the seller has left himself a lot of haggling room by asking $500 for it.
High Standard made a specific model of the Sentinel called the Sentinel Deluxe, and that is not one of them. I think it is a fairly early production gun judging by the grips and the front sight. It would have been helpful if the seller had put the R-series designation on the tag.
There is a good article about the Sentinel here, which explains the different varieties:
http://unblinkingeye.com/Guns/Sentinel/sentinel.html
It has no information about values, though.
PS - rcmodel is right, the frame is an aluminum casting, so the finish on it may not be nickel. Aluminum is usually anodized. High Standard made some guns with gold, pink or turquoise frames to take advantage of that, but they were not popular at the time. Collectors pay high prices for them now, though.