I don't disagree with the other advice here, and it is helpful to know that properties, on average, sell for 3%-5% below list (at least nowadays). And that's the number you were looking for. However...
Forget the nonsense about basing your offer on whatever the list price of the house is. And it is nonsense. Some houses are overpriced; some are underpriced. Some need repairs; some don't. Averages are fine if you're buying 500 houses; hundreds (or thousands) of house sales are averaged to produce your magic 3%. But, unless I'm wrong, you plan on buying one house. Not 500. So what you want to do is find a house that fits your needs and is a good value at or below your $350,000 budget. And that might be a house priced under market at $280,000, and would be a great value at that figure. Or it could be a property priced at $400,000, in which the agent may have painted a too-rosy picture to the sellers...and the sellers now are willing to get out for what they owe.
Also watch out for the hidden "seller subsidy" in many sales. There's nothing wrong or illegal about it; most loan programs permit it. But, for instance, a house might "sell" for $300,000, but the seller might credit the buyer with a $9,000 seller subsidy (to be used for down payment, closing costs, etc.) Thus, as a practical matter, the house actually sold (that is, net to sellers) for $291,000...even though the official records show $300,000. That's one reason why, even in this very slow market, some houses seem to sell at or even above their list price.
Again, remember: Knowing the averages are fine, but in buying (or selling) a specific house, look at your needs, look at the comps, look at the true value, and leave the averages to statisticians.