Progressive Past Tense
The progressive past tense (also known as the Past Continuous Tense):
The past continuous tense, also known as the past progressive tense, refers to a continuing action or state that was happening at some point in the past. The past continuous tense is formed by combining the past tense of to be (i.e., was/were) with the verb’s present participle (-ing word).
"Was standing" identifies an action that took place in the past that, from the perspective of the past, was still happening. "He was standing there when I left," meaning that the person continued to stand in the past forward to the present to the knowledge of the speaker.
Simple Past Tense
The simple past tense:
The simple past is a verb tense that is used to talk about things that happened or existed before now.
"Stood" identifies an action that happened in the past that was completed in the past. There is no expectation that the action continued beyond that moment of the past. "He stood there, then he left," is an example of an action in the past that was completed in the past.
Summary of the differences
The progressive past tense is used when you know the action continued beyond the time in the past you're talking about.
She was flying to Venice for a vacation when her brother won the lottery. (The act of flying continues beyond the act of winning the lottery).
The simple past tense is used when you know the action was completed in the past.
She flew to Venice for a vacation and didn't know her brother won the lottery. (The act of flying was completed without knowing about winning the lottery.)
And concerning your examples
The amount of sales was standing at $10000 in 1990.
The above sentence (progressive past tense) means the amount of sales in 1990 was $10,000 and continued to be $10,000 into the future beyond 1990 until something acts to modify the condition (the state of being $10,000).
The amount of sales stood at $10000 in 1990.
The above sentence (simple past tense) means the amount of sales in 1990 was $10,000 but it may not have stayed at $10,000. The value may have changed later.