Find an average sold price of an eBay search.Do a search for a product, check the "Sold listings" option under "Show only" in the left navigation.Run this script by pasting it into the console tab of the chrome developer tools.

eBay average price

function Stats(arr) {
  var self = this; 
	var theArray = arr || []; 
	
	//http://en.wikipedia.org/wiki/Mean#Arithmetic_mean_.28AM.29
	self.getArithmeticMean = function() {
	    var sum = 0, length = theArray.length; 
	    for(var i=0;i<length;i++) {
	        sum += theArray[i];
	    }
	    return sum/length; 
	}
	
	//http://en.wikipedia.org/wiki/Standard_deviation
	self.getStandardDeviation = function() {
	    var arithmeticMean = this.getArithmeticMean(); 
        var sum = 0, length = theArray.length; 
	    for(var i=0;i<length;i++) {
	        sum += Math.pow(theArray[i]-arithmeticMean, 2);
	    }
	    return Math.pow(sum/length, 0.5);
	}
	
	self.setArray = function(arr) {
		theArray = arr; 
		return self; 
	}
	
	self.getArray = function() {
		return theArray; 
	}
	
	return self; 
}

var listings = {};
var soldPrices = [];
$('.bidsold').each(function(k,v)
{
	if ($(this).find('.sboffer').length > 0) {
		return;
	}
	var listingId = $(v).closest('li').attr('listingid');
	var price = parseFloat($(v).html().trim().replace('$','').replace(',',''));

	listings[listingId] = {
		'price': price,
		'total': price
	};

	soldPrices.push(price);
});

//console.log(listings);
//console.log(soldPrices);
var stat = Stats(soldPrices);
var mean = stat.getArithmeticMean().toFixed(2);
var stdDev = stat.getStandardDeviation().toFixed(2);

var message = 'Mean: $' + mean + '\n';
message += 'Standard Deviation: $' + stdDev + '\n';
var goodDeal = 0.5;
message += 'Mean minus ' + goodDeal*100 + '% standard deviation (good deal): $' + (mean - (stdDev*goodDeal)).toFixed(2) + '\n';
message += 'Mean minus one standard deviation (great deal): $' + (mean - stdDev).toFixed(2);

alert(message);

This no longer works. You can use mine (admittedly it is not as elegant).