LocalCoinSwap uses market average rates from CMC (CoinMarketCap.com) for the recommended Dynamic market price.
If your ad is priced in a currency that is not USD (for example EUR), we will also use the USD/EUR price from openexchangerates.com to calculate the price in Euro.
These rates are taken every 5 minutes. Hence, the prices on posted trades are updated every 5 mins. However, as soon as a contract is initiated on the trade ad, the price is fixed for that contract irrelevant of how long the contract is open for and fluctuations in the market rate.