The Bayfield 25 is a cute little cruiser but it is quite possibly the slowest boat of it's size afloat. They are well-made and quite safe for a small family. If you are willing to deal with the performance issue, it could be a great little cruiser for you. The following link will give you an idea of the ASKING PRICES
of boats that have been advertised for sale recently. Note that they would have sold for less.
boat for sale-classifieds
The boat that you are looking at has a diesel, which was optional on those boats, and if it is in good condition with the trailer, then you should expect to pay north of 10K for her - but 15K is too much. Offer 10 and let them chew on it for a while.
Good Luck !