Stacy Levine, left, and Melissa Bragg shop at a Toys R Us in Atlanta on Black Friday in 2015. Serious deal-seekers are already planning their Thanksgiving weekend shopping.(Photo: Bob Andres, AP)

Shopping for Black Friday doorbuster deals is like gambling: You might hit the jackpot or you might walk away empty-handed, with a wallet to match.

If you want to play your cards right, plan ahead. Here are four things you should know before shopping Black Friday doorbusters.

1. Doorbuster deals start before Black Friday

It's become commonplace for retailers to launch Black Friday sales on Thanksgiving, if not earlier. Overstock offered shoppers doorbuster previews as early as October this year. Amazon has released new deals every day in November as part of its countdown to Black Friday week.

It's not just dates you need to pay attention to, but times as well. Dell staggers its doorbusters by the hour. For example, the Inspiron 11 3000 laptop kicks off the online sale at 6 p.m. Eastern time on Thanksgiving, while the Garmin Vivofit activity tracker goes on sale at 3 p.m. on Black Friday.

Check retailers' websites early and often to avoid missing the item you're after.

2. What you see isn't always what you get

Just because you spot a spectacular deal in a retailer's Black Friday ad doesn't mean the store will have it. Most doorbuster promotions are good only while supplies last, and quantities might be more limited than you think.

At some retailers, items are claimed before the sale officially starts; Best Buy stores typically hand out tickets for in-store deals a couple of hours before the doors open. And on Thanksgiving 2015, Sears sold a Kenmore refrigerator at a 52% discount but carried only about two per store.

Prices can be misleading, too. Past NerdWallet studies have shown that many companies inflate retail prices, making sale prices seem lower by comparison. And at places like J.C. Penney, deals can require extra effort. For instance, the Cooks single-serve coffee maker is $ 30 — after a $ 25 mail-in rebate.

3. If the store's out of stock, you're not out of luck

Most stores are quick to say "no rain checks," which means blink and you'll miss the bargains. But there are some exceptions.

Walmart's 1-Hour Guarantee lets in-store shoppers buy certain items that sell out within the first hour of the event at Black Friday prices and have them shipped to the store by Christmas. And the Sears Holiday Guarantee promises members of its Shop Your Way rewards program a partial reimbursement on select purchases if the department store sells the same product for less during the holiday season.

And stores aren't your only option. Plenty of retailers offer their in-store deals online or have online-only doorbusters. Costco, for example, will have several online exclusives Nov. 24 through Nov. 28. Just remember to check for shipping costs.

4. Return policies vary

Most retailers accept returns. Some even extend their return policies during the holiday season. However, there may be final sales, exclusions or conditions. Take Target, where the return period for drones, electronics and entertainment items purchased on Black Friday won't start until the day after Christmas. Make sure to read the terms carefully before buying.

MORE: What to buy, and skip, on Black Friday 2016

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Lauren Schwahn is a staff writer at NerdWallet, a personal finance website. Email: lschwahn@nerdwallet.com. Twitter: @lauren_schwahn.

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