An option to filter out offers by minimum price would be sufficient. Introducing price floors would decrease number of offers which the site doesn't have much yet.
What do you mean by quality of the offers? Maybe you just want to eliminate cheap competition which has better quality/price ratio?
There may be demand for cheap services in developing countries and there may be people willing to work for such money there. Wouldn't you want to receive cheap service yourself? Cheap doesn't always need to mean poor quality.
If someone wants quality work done he is probably aware he won't get it for 1 BTC, but maybe it would be enough for him.
Bitcoin was created for freedom of exchange and any limitations would hurt the market.