Jeff
When one pays PST (how often) is based on sales volume. For some reason, my last remittance covered about 8 months, the one before that 6 months and the one before that about 3 months. If you operate a retail store, I believe that you are expected to report and remit monthly but I know that varies even in the restaurant and bar business.
I think a good rule of thumb is, if you are selling sufficient quantities to be a Power Seller then it is probably safe to assume you are running a business and should be reporting your sales.
If you sell personal items as well as used ones you purchase tfor re-sale, you should either set up a different user id or keep clear records, well documented as to ownership.
Besides, there is nothing wrong with selling personal items. Lets face it, that stereo you bought in 1980 for $800 just sold for $25 on eBay so that would be a handsome business loss in my books. (Sorry I dont depreciate personal belongings).
When purchasing at garage sales or at flea markets, take along a receipt book and have the vendor complete it for you or just complete it yourself, noting where you made the purchase, the date and what you paid.
Remember, you can deduct your expenses for travelling to the flea market and other items as well (such as a portion of your home costs) so it can be a bonus for your business to become legitimate.
Malcolm