"my book values of my inventory items at my retail prices "
Inventory must generally be valued at cost.
To calculate your cost of goods (for example):
 Opening Inventory (12/31/2006) 100,000			  
 Purchases during 2007	         50,000 
 Available for sale 		150,000 
 Closing Inventory (12/31/2007)  80,000  
 Cost of Goods Sold 		 70,000
If your sales for the period were $120,000, your gross profit would be $50,000 ($120,000 less $70,000)
From your gross profit, you deduct your expenses such as:
 Advertising 
 Office 
 Communications 
 Travel (car) 
 Listing Fees and Commission 
 Occupancy 
 Bank Charges and Interest 
To arrive at your Net profit (line 135)
As far as car expenses are concerned (I own my car), I find it much easier NOT to depreciate but to use a flat charge of $0.50 per km for business use during the year.  If you use your car for business purposes for 5,000km, a $2,500 would be deductible. One must keep logs.
I was audited for the last two years and that simple method was accepted without question.
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