if you look at increase in the Cambridge dictionary, you will see [I or t] at the beginning of the entry: this means that increase can be intransitive (subject - verb) or transitive (subject - verb - object). The thing that increases is the subject when the verb is used intransitively and the object when it is used transitively. Look what happens to rail fares in these two active mood sentences:
Rail fares increased by 18% this year. - intransitive
South-West railways increased rail fares by 18% this year. - transitive
We can also make a passive mood version. In a passive mood sentence, the patient (what would be the object in an active mood sentence) moves to the front of the sentence, and the agent (the subject of an active mood sentence) can be omitted, or can be attached using a by preposition.
Because the object moves to the front in a passive mood sentence, we can't make a passive mood sentence with an intransitive verb - there is no object. So, we make it with the transitive form:
Rail fares were increased by South-West railways by 18% this year - with agent
Rail fares were increased by by 18% this year. -without agent
Looking at your sentences, the first is in active mood, using increase as an intransitive verb with sales
as the subject.
The second sentence is in passive mood: the agent is omitted, so we don't know who or what is causing the increase to happen, and sales is the patient.
1) When sale is used to describe the number of items sold, it is always plural. It is treated a collective noun, so it is used with a singular verb. See the Cambridge Dictionary
for confirmation of this.
2) In many contexts, it is OK to use from... to... for a range of dates, but in the above sentence (after increase) it is misleading, and could be taken to mean that 2000 units were sold last time, and 2005 units were sold this time. If you want to be completely clear that 2000 is the starting year and 2005 is the ending year, it would be better to say "between 2000 and 2005"
3) It is not necessary to put the before sales in the second sentence.
The corrected sentences are therefore:
Sales increased in the UK between 2000 and 2005 - intransitive, active mood
In 2005, sales was increased in the UK - transitive, passive mood