As an individual on eBay, you can sell up to a certain amount of items before you have to consider tax considerations or be considered a professional seller. eBay offers users the ability to post free listings each month, but the number may vary based on current promotions. Once you have exceeded these free listings, you can still sell items, but listing fees will apply.
The sales volume you can afford without registering as a professional depends on your country's legislation and the tax rules for selling used goods. In many countries, if your sales exceed a threshold or if you make regular sales that indicate a commercial activity, you may need to register as a professional seller and fulfill the corresponding tax obligations.
I recommend checking your local laws and eBay policies to determine exactly how much you can sell as an individual without having to change state. It is also a good idea to consult with an accountant or tax advisor to ensure you are operating in compliance with current laws.