If the parts and the seller are in Germany, and you are in the UK, you will have to pay VAT on the whole value. The only exception would be if the seller in Germany was a business that had registered with the UK government for VAT, but you said they are a private individual so this won’t apply.
In practice, most likely the courier will send you a bill for the VAT to pay, which they pass on to the UK government. They will also charge an admin fee for doing this. If you buy through a platform like eBay, they are required to make sure the VAT is collected.
The fact the goods are second hand or the VAT has already been paid in Germany makes no difference, unless the seller is registered with the UK govt for VAT.
You can ask the seller to put misleading information on the label. That would be illegal and could delay your shipment arriving as some goods are inspected if the values seem implausible.
I’m not a tax expert but I have bought second hand high value goods from within the EU and elsewhere and this is generally what happens.
I’ve also bought goods from larger EU companies that are registered for VAT with the UK govt and the process is much simpler, no additional VAT to pay and no delays in shipping.
Duty is different from VAT and rarely payable on most goods that are imported. It’s the VAT that costs extra.
I found this reference hard to read but does have some helpful information.
www.gov.uk