Groupon Inc. took inventory of about 95% of the products sold via its Groupon Goods marketplace in the second quarter, according to data compiled by daily-deal aggregator Yipit. That’s up from about 75% of the items sold on Groupon Goods in the first quarter.

Groupon launched Groupon Goods last fall as a forum to sell discounted products. The offering was a quick success as Groupon sold $2 million worth of goods in its first week. The daily deal operator reported that Goods surpassed $200 million in revenue during the second quarter and the marketplace is expected to sell between $600 million and $800 million worth of goods this year, according to an estimate by Scot Wingo, CEO of e-commerce services provider ChannelAdvisor Corp.

Groupon Goods has a diverse product mix that ranges from novelty items like a $12 “scalp-massaging shampoo brush” to higher-priced items like a $629 Simmons Beautyrest pillow-top mattress. Assuming that the items that Groupon takes stock of are representative of the marketplace’s overall product mix,  that would mean that Groupon-owned items will generate between $570 million and $760 million in sales this year, going by Wingo’s estimate. That would place Groupon among the largest North American online retailers. For the sake of comparison, Saks Direct is No. 38 in the Internet Retailer Top 500 Guide with Internet Retailer-estimated 2011 online sales of $748.6 million and Abercrombie & Fitch is No. 45 with online sales of $552.6 million.

For the items that Groupon does not take stock of, it charges a commission estimated at around 10% to 15%, says Sean Spielberg, data product analyst at Yipit. That’s similar to the charges on marketplaces like Amazon.com and eBay.

By taking inventory of the vast majority of products sold through Groupon Goods, the daily deal operator can account for the items on a gross, rather than net, basis. That means that it can record the whole sale as revenue, rather than the revenue that Groupon collects from the sale. That stands in contrast from the way it accounts for its sales of vouchers. Groupon typically takes about a 40% commission from the sales of its vouchers, says Spielberg. In that case, it can claim net revenue about 40% of the voucher’s purchase price.

Because the margins for Goods are much smaller than they are for vouchers, the growth of Goods could be concealing weakness in the company’s overall business, says Abe Garver, principal at the Focus investment banking firm. “Unlike a Groupon coupon where Groupon may be only able to say that it generated $2 from a $10 coupon, with Goods that it owns it can record a $10 widget as $10 even though the item may have cost Groupon $9.90 to buy,” he says.

Groupon declines to comment on the percentage of Goods that it owns.