colors on boot: white: cpu failure
grey: could indicate different contact problems or hw failures, basically the machine went through checking hardware but couldnt start the system anyway if i recall right
green: basically faulty chip ram but could be something else too.
the guy says it has been refurbished: good. i tend to believe that the fotos of the unit are fresh so i dont think the motherboard has been chemically damaged and they do not break in transport. i beleve you really have component contact problems.
you say you are getting green when pressing on buster. it could be that buster legs are bent. it might happen if it has been removed before and carlessly reseated. it isnt that difficult to pull this chip but normally pushing it deeper into the socket is enough. i think applyin pressure to buster you bent the whole board a little and the flaky contact should be sought elsewhere.
to eliminate further possible failure sources i would pull the daughterboard and the fast ram. examine the cpu card contacts, assure the jumpers under cpu board are seated correctly (they could got loose or lost, lol), check the roms. reseat the card.
now try to power on again. also you could take the chip ram out to assure that you get green screen on boot, then reseat it.
edit:reason: the seller might have exchanged ram without checking, and a4k are sometimes quite picky about the modules.