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.
Users are able to use prices from other sources and other pricing equations through the more advanced "Pricing equation" pricing method. More information on this can be found here.