hi @gold84
first a bit of background on buying/selling ram on EOS
its not like a the SBD/STEEM internal market or any exchange like gdax or binance... instead of buying and selling with another person you are buying / selling directly with the eosio.system contract itself.
so to answer 1) when you sell ram back to the system contract, you receive an amount of liquid EOS proportionate to the current spot price of ram. if spot ram price is high, your receive more EOS in return. if spot ram price is low, you receive less EOS in return. the spot price is determined by something called the bancor algorithm (whitepaper: https://storage.googleapis.com/website-bancor/2018/04/01ba8253-bancor_protocol_whitepaper_en.pdf)
to answer 2) after selling the RAM, you're EOS is liquid and thus available to do whatever you want with it
as for 3) if you sell all your ram, you might not be able to do anything with that account anymore (there's a minimum amount of ram needed to function). but no, there's nothing really stopping you from doing whatever you want with your EOS after your sell RAM. its not like delegation where you have to wait 3 days.
hopefully that was clear, if not, please ask
RE: Live EOS Ram Price Chart and Buying Calculator