Value tells you what an item is worth. Demand tells you whether anyone will give you that worth today. They are different numbers and they fail in different ways.
The trap
You trade three easily-moved items for one high-value item with demand of 3. On paper you broke even. In practice you now own something you cannot trade for weeks, and every offer you receive is a lowball because the other side knows it.
When to take low demand anyway
Only when the item is genuinely scarce and its value has been climbing steadily. Scarcity plus an upward trend eventually creates demand; scarcity on its own does not.
A simple rule
If you plan to trade the item again within a week, never drop below a demand of 6. If you plan to keep it, demand does not matter at all.