Theory:

THE PAST - PUTTING ALL THE DIFFERENT FORMS INTO CONTEXT

Look at the following situation:

1. I am selling tickets now.(PRESENT CONTINUOUS)
2. I sold 5 tickets yesterday.(PAST SIMPLE)
3. I was selling tickets when I saw Maria yesterday.(PAST CONTINUOUS)
4. I have been selling tickets for 2 hours.(PRESENT PERFECT CONTINUOUS)
5. I have sold 5 tickets.(PRESENT PERFECT SIMPLE)
6. When you came, I had been selling tickets for 3 hours.(PAST PERFECT CONTINUOUS)
7. When you came I had sold 8 tickets.(PAST PERFECT SIMPLE)

Sentence 1 is the necessary condition for sentences 4 and 5.
If sentence 1 is not true, sentences 4 and 5 are not valid.

For sentence 2, we only know the result of the activity and that when it happened is not connected to NOW.
We do not know the time taken to sell the tickets or anything else.

For sentence 3 we know 2 things:
At a time which is not connected to now, two things happened at the same time, SELLING TICKETS and SAW MARIA.
We do not know for how long I was selling tickets.
We do not know how many tickets I sold.
Sentence 3 only describes what was happening at the moment of interruption when selling tickets and saw Maria happened at the same time.

Sentence 4 tells me the duration of the activity of selling tickets, 2 hours.
It also tells me that the activity started 2 hours ago and continued until now.

Sentence 5 tells me the result of the activity that started 2 hours ago and continued until now.
I have sold 5 tickets BECAUSE I have been selling tickets for 2 hours.

Sentence 6 tells me that I started an activity 3 hours before a fixed time in the past, and continued this activity until that fixed time in the past (WHEN YOU CAME IN)
Sentence 7 tells me the result of sentence 6, with all the same conditions.