The formula
price = price when new × (1 − drop)^years × condition factor
Where it comes from
Things do not lose value in equal slices each year: they take a big bite the first year and the fall softens after that, because what depreciates is always what is left. That is the difference between a straight line and a geometric curve, and the reason a two-year-old phone is not worth exactly half of a one-year-old. The yearly drop depends heavily on the category: consumer electronics runs around thirty to forty per cent a year, appliances fifteen, good furniture and tools considerably less. Condition applies afterwards, on what is left, and the box or a live warranty are a small but real premium at resale.
How to work it out by hand
- Apply the yearly drop as many times as the item is old: that is the value from age alone
- Multiply it by the coefficient for the condition it is in
- Adjust upwards if you kept the box, the accessories or a warranty
- Subtract what it would cost to fix whatever does not work
What is worth knowing
This gives a starting point, not a market price. What settles it in the end is what that exact model is actually selling for, and that is read in sold listings, not in live ones: anyone can ask whatever they like. If you have that reference, put it in the market price field and the page will tell you whether your figure fits.