What matters is your intent, not how much profit you make or how long you are involved in the business.
If you are engaged in an ongoing trade or business, and you conduct your affairs in a businesslike manner (you advertise, you seek out opportunities to buy and sell items, etc) then you are probably a business, even if small.
It does sound like this should be reported as a business.
Reporting as a business means you pay income tax on the net profit and also self-employment tax (an extra 15%). But it also means that you can deduct certain expenses that are not deductible if the activity is a hobby, and you get credit towards your retirement or disability with the social security administration. (You may have other tax benefits open to you depending on your other circumstances, like a self-401k or health insurance premium credits.)
