How Much to Offer

To answer that I'd have to know about the seller's asking price.  Some homeowners tack on more than they'd really take, hoping lightning will strike, or because they're not really motivated to sell in a hurry anyhow.  Some sellers hate bargaining, or are in need of a quick sale, and list at a rock-bottom price.  There's no general rule; every situation is different.  But local brokres who belong to a multiple listing system have the data to tell you the average ratio of sale price to asking price on recently sold properties.  

Remember, if sellers also realize your area has a down market, the wise ones have already taken that into consideration in their pricing, if they expect to attract potential buyers.  You might do better basing your offer on an analysis of sale prices recently in that neighborhood.