1. Do I have to file this income in my yearly tax declaration?
2. Is there any chance the authorities will ask me to justify this income i.e. provide some form of proof?
Recently I have sold some no longer needed and rather expensive items (all second-hand) via Facebook Marketplace e.g. electric bicycle, Apple laptop etc. amounting to almost ~3k CHF. I accepted cash. Since I prefer electronic payments in my day-to-day life I am thinking to cash deposit this amount into my bank account.
No need to declare - this is not income. Don't think anyone will notice this size of amount.
The question is whether this activity is a side business or a hobby. BDO writes this (translated):
"Who decides whether a hobby or a self-employed side business exists?
Whether a “tax-free” hobby exists or not depends on the circumstances of the individual case.
A commercial business is assumed by the tax administration if intensive, regular trading takes place, in which the purchases and sales are made in a way that goes beyond the simple management of private assets and the clear intention of making a profit is recognizable. Often, outside capital is used and the seller visibly appears on the market. It is always particularly critical if the sideline activity is related to the main profession."
3k will not even register. Anyway, it'll be part of your wealth tax calculation at the end of the year.