drug stores like Müller, DM, etc., also a couple of weeks before the school year starts (usually in September) Discounters like Aldi, Lidl, Netto, Norma, Rewe etc. offer office supplies really cheaply
Used Goods (e.g. Furniture, Bicycles, etc.)
If you agree on a trade online never agree to pay in advance without some sort of buyer’s protection (Paypal etc.). I you can, it is best to pick up the good yourself and pay in person.
buy, exchange or even get free used stuff in Erlangen und other cities in the area: Verschenkbörse