I notice a lot of reviewers buy the sample piece

When I read Absolute Sound or Stereophile I notice quite a few reviewers will buy the sample gear. Do they get the piece at dealer cost or even below that? I wonder if they have to keep it for a period of time before they can resell it?

My understanding is that it could be a combination of both--what is known as an "accommodation" price and a commitment not to resell for a period of time.  I use the word "could" deliberately because I am fairly certain there my be some manufacturers that do not offer accommodation pricing.  Likewise certain reviewers and/or publications might disallow this to maintain journalistic integrity.

They often use that as the ultimate endorsement. I have read in many places that they get a great price, but there is a time limit before they can sell. 
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The reviewers get an "endorsement" discount. I feel confident that it is at a cost or below price.
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Industry accommodation pricing, which is below dealer cost (dealer cost typically is 60% of msrp). 

Includes a a commitment to keep the gear for a period of time, at least a year. Some gear is shipped to reviewers under extended loan. 

Not it all reviewers are Boy Scouts, though.

Buying the gear is a critical endorsement-.ie, putting your money where your mouth is, but it's not entirely genuine. The phono cartridge which just got a rave may cost you $5000 but it might cost the reviewer $2000. 
not only is the purchase an endorsement, the item is then listed as part of the reviewer's reference system for a year, which is also great advertising...
If there is a competition for new threads by a Audiogoner, 'taters' would have won it in a heartbeat....LOL!!!

What a scam. I feel pretty naive as I have always considered the reviewers decision to buy a reviewed piece like someone said above as the ultimate endorsement. I see now the reviewer has no real skin in the game and any authentic endorsement implied is dubious at best. 

It's an endorsement but not the ultimate unless the list price is paid.  

Cmon this is America where money always speaks loudest. .  Could reviewers on the take possibly surprise anyone. 
Arthur Salvatore has a lot to say on this subject on his Reference Components website.
reviewers also tend to only review equipment they have heard and liked. no one wants to listen for a few months to gear they do not like.
out of curiosity I did a quick random check, and noticed only a small percentage buying the reviewed equipment...

read between the lines...

when the reviewers mentioned they received the component from previous reviewer or it has been burned in already or whatever tell-tale signs, all kinds of endorsement, "highly recommended", their purchases are simply their marketing gimmick. Reviewers whom can honestly compare between similar gears in a controlled audio setting, highlighting strengths & weaknesses between components are rare these days.

One of my audio buddies (who used to own a high end dealership) said he knew one of the reviewers from the major publication who used to tell him that some manufacturers offered the samples at a huge discount, better price for a better review. He said that, flipping the review samples offered the reviewers a little bonus to their income.
Back in the late 70's I worked at some high-end dealerships in the DC area.  I'll never forget how a speaker from a famous Japanese company in the $750/pair price range was positively reviewed in several publications, but when customers would compare them to speakers from other manufacturers, they NEVER bought the Japanese model, NEVER.  There was nothing wrong with the speaker, it just wasn't at all competitive in its price range.