Some Merchants prohibit their customers from reselling products. If the Merchant's website states "Not for Resale" or similar language, you should not order products from that Merchant with the intent to resell them. You should also honour any other terms of sale specified by the Merchant.
Some countries have laws or regulations restricting the resale of imported products. You need to check the laws of your country and comply with those restrictions.
If you do buy products for resale, then depending on the country where you are located you may need special VAT or import tax registration or licenses. Please check your country's import requirements. Shipments that appear to be imported for resale (e.g., multiple copies of the same or similar items) may be delayed by customs if there is no required resale permit or license.