@JulienJ 
 
You will need to create a journal entry to remove the asset, close out the accumulated depreciation taken, record the A/P vendor credit (debit entry in QB), and book the difference to gain/loss.  For example, let's assume you had a fixed asset with an original cost of $10K, you took $5K in depreciation and you sold it to the vendor for $7.5K.  The journal entry to record that is: 
 
|  | Debit | Credit | 
| Accumulated Depreciation (to close the amount taken on fixed asset) | 5,000 |  | 
| Accounts Payable (vendor credit to be applied to future bills) | 7,500 |  | 
| Fixed Asset (to close) |  | 10,000 | 
| Gain on sale of fixed asset - other income to balance (debit if loss, credit if gain) |  | 2,500 |