What is the current average daily trading volume for Monero on known exchanges?
How many known exchanges trade Monero?
Is there any way to estimate the amount of P2P trading that takes place but is not published?
You can see the total daily volume on non-p2p exchanges and broken down market here:
Over the last 24 hours the total volume was about 316 BTC or 207k USD, with most of that concentrated at poloniex.
There is one p2p trading service that you can check the volume on - bitsquare, but I don't think they're doing more than $100 per day at the moment.
As far as I know there is no way to know how much OTC trading is occurring on any given day, except maybe trying to estimate by lurking in #monero-otc channel in freenode.
Most of the volume $100k+ is on https://poloniex.com/exchange#btc_xmr
There is like $1-10k volume daily over on https://bittrex.com/Market/Index?MarketName=BTC-XMR
You can see a quick total daily volume at http://coincap.io/XMR
or for more details check out http://coinmarketcap.com/currencies/monero/#markets