save up some cash, buy some stuff cheap, sell not that cheap, repeat until satisfactory.
good tip is - learn about popular defects like dead batteries and stuff that make keyboard appear nonfuctioning. this way you can buy a supposedly 'dead' keyboard for 30 euros and sell it for 300 after replacing a 1eur battery or something. like dx7, poly800, etc. unless you were unlucky and it really was just dead. a guy i know bought a very rare crumar for 100 euros that was 'untested' and it turned out there was no electronics inside it.
be smart, generally speaking.