What expenses are deductible when selling on ebay?

Last year I had made about $2,500 selling on eBay (eBay sent me a 1099).  This is a hobby as I have a regular full time job in addition to eBay.  Can I deduct such things as printer cartridges (I print postage), packaging supplies, and costs of the items I sell which I buy at thrift stores, book sales or estate sales?   And if so, how can I prove costs of items from estate sales since it’s almost always cash to an individual?  Is keeping a log of dates, places, items and costs sufficient?  I may have made $2,500 but it certainly wasn’t all profit.   Thanks so much - and any additional suggestions are also welcome!

~ Charlene