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In the expanding world of online shopping, flash-sale websites are all the rage. These sites offer a variety of merchandise at heavily discounted prices. The catch? Sales usually last just 24 or 48 hours and product availability is limited. Items often sell out within hours, and customer complaints about service and shipping dog some of the e-vendors. Here are four best flash sale websites, ranked according to reviews and range of offerings.

A quick note: The flash-sale sites discussed below offer deals to "members only," but there's nothing exclusive about them. Just provide your email address and you're welcome to browse and shop as you please.

1. Gilt Groupe.

Gilt Groupe is a veteran among daily deals websites and one of the largest. Gilt sells discounted goods for women, men, kids, and home, as well as city deals for restaurants, retail shops, services, and events, and travel deals for hotels, airlines, and package offerings. This flash-sale site recently launched an "insider points program" that rewards loyalty with redeemable deals for a given number of points. Members get 500 points for joining and accumulate more by interacting with Gilt, which means buying something or just visiting the site. Points expire after six months if no purchases are made.

User reviews of Gilt are discouraging. It earns an average two stars at SiteJabber based on 152 posted comments. Many complaints focus on the return policy and shipping mishaps; consumers gripe about damaged or incorrect shipments and unhelpful customer service. Users also seem miffed about store credit as the only type of refund. Reviews at Reseller Ratings echo these assessments, and while the number of posts totals less than two dozen, the average rating for Gilt sits at 1.5 out of 10. Gilt fares better with the Better Business Bureau (BBB), which assigns a B rating to reflect the site's responsiveness to complaints.

The company's 21-day return policy states that items costing up to $199.99 and not marked "final sale" can be returned only for store credit. Refunds are offered on higher-priced items. Gilt will refund the $7.95 return shipping fee if the item is damaged or shipped incorrectly.

Verdict: Although flash-sale sites reviews are mostly critical, Gilt is more widely reviewed that other sites we researched, and it holds a passing B grade from the BBB rating. The major attraction at this site is the large selection of deals in the areas represented. The new Gilt Insider is both a perk and an incentive to buy more. Overall, the occasional mishaps are outweighed by the benefits.

2. RueLaLa.

RueLaLa offers an expansive selection of merchandise. In addition to men's, women's, and kids' categories, there is a living section that includes home decor, home appliances, and food. A travel tab rounds out the options and offers experiences similar to those available at Gilt. RueLaLa maintains an extensive blog organized around topics such as fashion, living, and beauty and offers two plans that reward frequent shoppers. Rue 30 lets customers qualify for free standard shipping on all purchases for 30 days after paying the $9.95 shipping fee on one order. Rue 365 is a membership plan costing $100 a year that offers free shipping and two $50-off cards for brands the shopper selects.

The average rating from more than four dozen reviews at Site Jabber hits 2.3 out of 5. Customers grumble about poor service, particularly when the issue concerns defective items. Other complaints mention problems enforcing product warranties. Still, the flash-sale site garners an A rating from the BBB.

Returns are free in exchange for store credit, otherwise shipping costs $9.95. Except for final sales, returns must be made within 30 days.

Verdict: Like Gilt, RueLaLa wins points for its expansive selection across various categories. The Rue 30 and Rue 365 plans are welcome perks for frequent shoppers. And reviews are about on par with other flash-sale sites.

3. JackThreads.

JackThreads, a men's deals site owned by Thrillist, carries designer labels but focuses more on street and urban wear. Retail categories include apparel, gifts, home decor, and gadgets/electronics. The FAQ notes the difference between "in-stock" items and "pre-sale" items, with estimated delivery times of 5-11 business days and 3-4 weeks, respectively.

Reviews of JackThreads are mixed. The flash-sale site earns an A- from the BBB for the relatively small number of complaints received in the past three years and how the company responded. Trust Pilot, an online-shopping review site, gives JackThreads an average rating of 5.7 out of 10 based on 16 reviews. The majority of consumer complaints relate to slow service and the site's return policy. JackThreads takes heat in reviews at Reseller Ratings, where it earns a score of 3.5 out of 10 for issues such as slow shipping, questionable product quality, and poor customer service.

The return policy stipulates store credit for returns of non-final-sale items. A $5.95 fee is levied when using the supplied return-shipping label, and the return window remains open for 14 days.

Verdict: JackThreads would rank higher based on user reviews and its BBB rating, but merchandise is limited to men's apparel. It earns a spot on our list anyway due to quality products and customer service that slightly bests other sites.

4. HauteLook.

HauteLook is owned by Nordstrom and sells luxury brand names in women's, men's, and kids' apparel, as well as in home decor and beauty, the latter a category rarely seen on other flash-sale sites. Another feature of the site is "Nordstrom Rack for HauteLook," which offers 24/7 sales that do not expire, shipping within 1-3 days, and returns to any Nordstrom Rack store.

HauteLook reviews at Viewpoints trend negative (the deals site claims a rating of 36 out of 100 based on 54 comments). Many complaints center on less-than-stellar customer service, long shipping times, and a general lack of quality products. At Site Jabber, 154 reviewers award the site an average 2.3 out of 5. Again, most of the grousing concerns slow delivery.

All orders greater than $100 qualify for free shipping. For smaller orders shipping starts at $7.95. HauteLook says that orders shipped within the U.S. usually take 7-10 business days. There is no expedited shipping.

Returns are accepted within 30 days of purchase, although some items marked as final sale or vintage cannot be sent back. After that period, HauteLook reserves the right to accept a return but will assess a 50 percent restocking fee. Using the site's return shipping label costs $5.95.

Verdict: HauteLook's most customer-friendly feature is the option of returning items from the "Nordstrom Rack" section to any Nordstrom Rack store, which means bypassing the return shipping problems that plague other flash-sale sites. That said, and despite the decent deals, HauteLook doesn't offer many incentives to keep shopping.