Start from your full cost — the hammer price is not your purchase price.
Start from what the piece really cost you: the purchase price plus transport, any import costs, and whatever you put into repair. Then account for the VAT that sits inside your margin, because you pay that over. What is left is your real profit — and only then do you compare with what similar pieces fetch.
What the piece really cost you
Start from that total, not from the hammer price alone.
The classic miscalculation is putting the VAT on top of the margin instead of inside it. On a margin of 100 euros, Belgium takes 17.36 euros of VAT. Ignore that and your profit is a sixth smaller than you thought.
The VAT sits inside your margin, not on top
A margin of 100 euros, in Belgium100 euros
What is left
82.64 eurosVAT you pay over
17.36 eurosIgnore it and your profit is a sixth smaller than you thought.
With a unique piece there is no comparable price, only a comparable piece. When you set the price, note why you chose it; a year later that is the only thing that helps you decide whether to mark it down.
In Belgium the standard rate of 21 %, and this is where it usually goes wrong: the margin is a VAT-inclusive amount. You calculate margin × 21/121, not margin × 21 %. On a margin of 100 euros that is 17.36 euros, not 21 euros. The Netherlands also uses 21 %, France 20 %, and in the UK HMRC itself calls it one sixth of the margin.
Work out why it is not selling before you mark it down. Is it at the back of the shop, are there no good photos, is it not online, or is the price genuinely higher than the market gives today? Only once the first three are right is the price an honest answer.
Vintro Pro keeps, per piece, what you paid, when, and where it came from. Your stock list and your margin VAT build themselves while you work.
30 days free, no card number.