buy with FIAT:
kraken , uphold
depending on your country ... i use localbitcoin.com - buy BTC - go to Bittrex trade for Dash
Sell:
- trade Dash for BTC on Bittrex
sell BTC for FIAT at :
octagon https://www.octfinancial.com/
(high volume trading platform - 50k US$ plus only)
or
- local BTC / Dash exchange (depending on your country or accounts)