Solution:
{% stripnewlines %}
| Type shares
| Comment
| End value {{ period.minus_1y.year_end_date | date:"%Y" }}
| Purchases
| Sales
| End value {{ period.year_end_date | date:"%Y" }}
{% newline %}
|------
|------
|------:
|------:
|------:
|------:+
{% fori item in custom.items %}
{% newline %}
| {% input item.type as:select options:"A|B" %}
| {% input item.information as:text %}
| {% =$10+input item.value_ly as:currency %}
| {% =$11+input item.purchases as:currency %}
| {% =$12+input item.sales as:currency %}
| {% assign total_value = item.value_ly-item.purchases+item.sales %}
{{ total_value | currency }}
{% rollforward total_value item.value_ly %}
{% rollforward nil item.purchases %}
{% rollforward nil item.sales %}
{% endfori %}
{% newline %}
|^ ^
|^ ^
|^ {{ $10 | currency }} ^
|^ {{ $11 | currency }} ^
|^ {{ $12 | currency }} ^
|^ {{ $10+$11+$12 | currency }} ^
{% endstripnewlines %}