Prepare photos, serial number, condition and accessories

Before offering your item, collect the same material you would want to receive from the other party:

• overall photos;
• details;
• serial number;
• defects;
• modifications;
• functional condition;
• included accessories;
• available documentation.

This creates a symmetrical basis for negotiation.

If you expect transparency about the other item, be prepared to provide the same level of information about yours.