fatklass,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<table class="vis" id="own_offers_table">
<tr class="offer_container " id="offer_49051" data-id="49051" data-count="2" data-wanted_wood="150" data-wanted_stone="0" data-wanted_iron="0"></tr>
<tr class="offer_container " id="offer_49052" data-id="49052" data-count="1" data-wanted_wood="0" data-wanted_stone="150" data-wanted_iron="0"></tr>
<tr class="offer_container " id="offer_49053" data-id="49053" data-count="1" data-wanted_wood="150" data-wanted_stone="0" data-wanted_iron="0"></tr>
</table>
<script>
window.addEventListener('DOMContentLoaded', function() {
var nodes = document.querySelectorAll('.offer_container'),
sum = [].reduce.call( nodes, function(sum, tr) {
return sum += tr.dataset.count * tr.dataset.wanted_wood
},0);
alert(sum)
});
</script>
</body>
</html>