"I wasn't aware that the CRA had mandated that eBay share Power Seller information to them."
That is irrelevant. All Canadians need to report their income, even small business income, on their annual tax return. It does not matter if the income is generated online through eBay or other venues or through brick and mortar stores or flea market stalls or whatever. Everyone must declare their net profit (sales minus cost of goods minus related expenses).
$300 to $400 to prepare the bookkeeping and the tax return is reasonable. Personally, based on your number of transactions last year, I would charge more than that.
On the other hand, you can do it yourself, using speadsheets (MS-Excel or similar programs). In different spreadsheets simply compile your sales, your purchases, your business related expenses.
Many years ago I prepared a sample Profit and Loss Statement that can be used for income tax purposes. Please take a look at this http://www.pierrelebel.com/lists/P&L-sample.htm
Canada Revenue Agency offers similar forms.
It all depends on the value you put on your time. $400.00 the first year may be a bargain as understanding all the tax rules pertaining small home based businesses is challenging.
Good Luck.