I only use the float test to filter out the obvious ones. I find the drop test more useful. That said, the drop test wasn't so useful when I started collecting because i had no point of comparison.
IMO the sound difference is clear as day, and usually repro blasters especially just feel wrong in hand. The weight often feels wrong to me too so I wondered if this was something that had been looked into? Although perhaps it's more to do with texture and brittleness than weight :?:
I guess the problem is that it's often impossible to tell from most eBay pics. Unless the seller is A1 reliable for job lots I tend to just assume everything is fake and only offer what I'd offer for loose with the worthless repro. Then anything that happens to be authentic should I win is a bonus... possibly sad, but true!
Without wishing to throw a grenade into the discussion, I do feel like it's a shame there isn't a 'reputable' form of repro (e.g. particular marking / colour) so people with loose collections but not silly money could 'restore' their own toys without fueling the repro trade. Personally I'd rather leave them loose but I do appreciate why some might want to do that, and I do think it would be fundamentally different to those who use nigh on impossible to spot repro to dupe the unwitting on eBay.