Transaction charges are paid when transacting, not when buying and selling, or not less than not essentially.
Assuming you are buying and selling on an change website, you should have an account there. You should purchase and promote BTC for different currencies, however all that’s occurring is that the change credit your account with what you purchase, and removes it from the vendor’s account. This course of doesn’t contain any interplay with the Bitcoin community; it is all simply updating numbers within the change’s database of accounts.
The change could cost buying and selling charges for providing this service, however the particulars of that will rely on what website/service you employ, and the quantities concerned. Some exchanges provide a number of payment tiers primarily based in your buying and selling exercise.
There are transaction charges concerned when you determine to withdraw your BTC from the change to a pockets you management your self. The change could eat these for you, or they might cost you for them. Some exchanges could themselves cost a withdrawal payment on high of the BTC community transaction payment. The transaction payment is unbiased of the quantity transferred; it will depend on the complexity of the transaction, and that will depend on how the change chooses to dispatch their withdrawals as Bitcoin transactions.
As soon as BTC are in your personal pockets, and also you wish to use them, you may be the one to pay transaction charges. Typical transactions are round 150-250 vbytes in dimension. At 20 sats/vbyte, a 200 vbyte transaction would value 4000 sats or 0.0004 BTC. Once more, this worth is essentially unbiased of the quantity transacted.