USD to Bitcoin conversion for XMR trade

Hi,

First time buyer, looking for some guidance. As a step into using LocalMonero, I’ve initated a trade for 50USD to XMR (using Bitcoin as payment method).

How should the 50USD to BTC be calculated? Is this on the seller to provide the BTC amount or do I just calculate based on he current rate? It seems with out an agreement on the BTC amount there could be disagreement on the BTC amount.

One other quick one - the seller has asked for a tip - is this normal?

Thanks

The market rate is generally gotten from coingecko.com

As for the tip, it’s not prohibited to ask for a tip, but, of course, you have no obligation to leave it and almost everyone doesn’t, so don’t worry about it.