While the pdf you included is not for a furniture asset account and thus does not explain either purchase or sale of these assets, here is what should happen.
Purchase of furniture can be handled one of two ways. If you are always replacing furniture, say as often as replacing appliances (on existing or new units) you can utilize the Safe Harbor de minimis rules that allow for up to $2500 each line item of shorter life assets to be direct expensed and not even show on the balance sheet. Then when sold or disposed of the entire price received is pure profit.
The other way, which you may be more familiar with, an asset is purchased and is expensed over its IRS declared life expectancy (furniture in general is a 7 year item and in straight line depreciation you take a half year the year it goes in service). So you purchased furniture, and may have taken deprectiation, now it is being sold at a value less than you paid, but possibly more than its value on your balance sheet.
Say you paid $7,000 for furniture 3 years ago, and have taken depreciation 3 times, that would be 500+1000+1000 or $2500 in depreciation and now its value is $7k less $2.5k or $4,500. If you sell it for $5,000 (which is less than you paid) you have a profit of $500, if you sell it for $4,000 you have a loss of $500
