This is a really good question. I heard BasicSwap uses CoinGecko which in turn uses CEXs best way may be to just turn off that feature entirely and let the order book decide. Though, Bitcoin has an extremely low chance of ever being delisted, so you could use bitcoin CEX ptice and let the order book determine how much BTC its worth (like this coin trades for 500k sats). then 500k sats is fiat price so display what 500k sats is worth as that coins price