With a floating price, the number of bitcoins purchased fluctuates along with the bitcoin exchange rate. The amount of the transaction in bitcoins is determined at the time of closing, not the beginning of the transaction.
The floating price is calculated based on the exchange rate variable LocalBitcoins sell. The seller can also set an additional premium.
How do I specify a dynamic price?
You can look at the price setting guide for using more complex formulas. If javascript is enabled in your browser, then you can use weighted (USD24) or the market price (bitsmap) as the base price and enter your commission percentage in the “commission” field.
You can also use a negative commission value if you want to sell BTC below the market price (or pay extra).