What matters here isn't really the amount, but how the tax authority will interpret it. Legally, if you're selling personal items that you actually used yourself, you don't owe any tax. But "six months of regular sales" sounds like a business activity, and inspectors will notice that. They'll look at how often you post listings, whether the items are new (or clearly resold goods), and whether you have any trading experience.
The hardest part here is proving that it's actually your old stuff and not purchased inventory. Keep screenshots of your listings where you honestly describe the condition ("used," "scratches," "worn"), - that works in your favor. If you still want to be safe and file a declaration: you can fill out a Form 3-NDFL and list the income as "profit from the sale of property," there's a special line for it. When selling personal items, the tax rate is usually lower or can be zero under certain conditions (for example, if you owned the item for more than three years).
Life hack: if you start getting letters from the tax authority with questions, don't ignore them. Better to call your local tax office right away and ask whether you need to file a return in your specific case - they'll give you a specific answer, and you'll have a documented position. Plus it shows good faith.