I've never used Scottrade, however normally lots are determined by the FIFO method (first in, first out), so, if you bought the $10 shares first then the $6 shares, your first sale of one lot would generally be applied to the $10 shares. Once you sell one lot, you can buy shares back again later at any price and the next sale will be the lot you bought at $6. Be aware, however, of wash-sale rules if you sustain a loss.
There may however be restrictions on a rebuy due to various regulations (account type cash or margin, account balance <$25K, pattern day-trader status, etc.) -- check your broker for details on your particular account.
Newly
