That would be soft spot on the hull. I'd run away from the boat, very fast.

You can inject epoxy into the decks to get them hard. WIth that said, I wouldn't pay more than $500 for a boat with soft decks. Period. Offer $300 and tell the seller that the decks are soft.

Soft spot repair seen here.
http://www.catsailor.com/forums/sho...2&an=0&page=1#Post138682