But the website says: Your Bitcoin will not be exchanged if the obtained value will be less than 5% of the market value. If it happens you can still exchange it by yourself on BTC-exchange.com
It means, that if our market's price will change >5% (in between (minutes usually) your submission time and actual processing time), the exchange operation will not be processed. This happens very rarely.
I agree it's a weird sentence. I think what is means is that when you send BTC to that address it will automatically be sold as a market order on btc-exchange.com however as a protection for you they will not accept a sell order below 5% of the current price. In that case you will instead get the deposit at btc-exchange.com and you may manually place a limit order at whatever price you choose.
Yes
