From Quicken's trading site (see the last paragraph):
"Short sales during extended-hours trading
For Exchange-listed securities, the uptick rule applies, as does the rule requiring an affirmative determination that the security is available to borrow.
The uptick rule is applied against the Consolidated Quote System (CQS) closing / last trade prices, not the NYSE or AMEX closing price. The CQS reflects the last trade from all market centers trading listed stocks and disseminates this information until 6:30 p.m. ET. Archipelago quotes (the system used by Quicken Brokerage powered by Siebert) represent one of many participants which collectively disseminate last trade information to compile the CQS data. As a result, the Archipelago quote alone may not reflect the actual price at which a short sale may be executed.
For pre-market trades, the closing CQS price from the previous trading day (i.e. 6:30 p.m. ET) is used. For example:
1. CQS close for IBM was 99.50 on a down tick.
2. In the pre-market session the following business day, the current quote for IBM in Archipelago is 98.25 x 98.45.
3. If a limit order is placed to sell short at 98.25, Archipelago will automatically reflect the order on the offer at 99.51 (.01 higher than the CQS close).
4. The order will only receive an execution if IBM trades at the price of 99.51 or higher.
Short sales for Over-the-Counter (OTC) securities and Exchange-Traded Funds (ETFs) can also be accepted during extended-hours sessions, but the uptick rule does not apply for these securities, so OTC stocks and ETFs may be sold short without reliance on the closing bid. An affirmative determination that the security is available for borrowing must still be made for OTC securities and ETFs."