Although my answer is similar to that of the other respondent, I would like to explain its meaning in a little more detail. In this context, “short” means a selling side who aims to take a profit out of a bearish market (= short market), and “建玉 (tategyoku, or tachigyoku)” refers to a sales-purchase contract, or an account, concluded at agreed price between a seller and a buyer, which is usually made at financial futures or commodities exchanges (such as LIFFE, NYMEX, and NYBOT) or the like.

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Note added at 17 hrs (2010-04-08 07:16:05 GMT)
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There is no doubt that the other answers and references are secondhand information from the Web. The information I have provided is based on my hands-on translation experiences with the commodities and futures markets. The following are the reliable info supporting my view:
◆　建玉（たてぎょく）：　　　取引所で売買約定したもの。「たちぎょく」という場合もある。売りは売り建玉、買いは買い建玉という。〔出典：　商品相場用語辞典　同友館〕
◆　玉（ぎょく）：　　取引所で売買約定をしたもの。買い約定したものを買い玉、売り約定をしたものを売り玉という。〔出典：　商品相場の実際　同友館〕
◆　売り玉（うりぎょく）：　　商品取引所で売買するものの売り建てたもの。ゴム、生糸などと商品ごとに区別せず、売り建て買い建てをする一枚、二枚そのものを「玉」という。発音はギョクでタマとはいわない。すし屋がにぎりの材料をネタといい、古物商などが売買するものをブツというたぐい。反対は買い玉。〔出典：　商品相場用語辞典　同友館〕
Some pieces of the information described below are excerpts from my own database:
◇　short 売建
　short (...) 空売り（の～）：　先物相場での売り方。先物価格が下がると信じ、それによって利益を得ようとする側。←→ long (...)
◇　short account 売玉
◇　long account 　買い玉　←→　short account
◇　long 　買い持ち（筋[の～]）、強気（筋[の～]）、買い（の～）、買い建玉。
◇　long interest 買い建玉。取組高のうち買いの側。強気筋の保有建玉。
　　　※　commitment or open interest　　　取組高
◇　position 建て玉

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Note added at 1 day1 hr (2010-04-08 15:44:28 GMT)
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To JpBaugh:
I think that makes sense.

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Note added at 1 day2 hrs (2010-04-08 16:17:55 GMT)
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Oh,there is "open" in your sentence, which is I think redundant.