There are a great many markets in the world, not all of them are large and/or efficient, and, on top of that, the diversity of personal investors is much greater than the diversity of markets.
'The', not 'a' or 'any'. Again, I chose to leave out any disambiguation and disclaimers beyond the pronoun choice, to avoid wordiness and because I thought no reasonable reader would misinterpret the statement. My instinct for pedantry that I so cruelly suppressed as 'unduly socially awkward' while writing my comment is now jeering "I told you so" at me. While such precautions come with a cost they at least serve to prevent this kind of exchange from time to time.
Just rename your instinct for pedantry "preference for clear and precise expression" and hoist your freak flag :-) Socially awkward on LW is a weird beast anyway :-D
To return to the original point, how about "Timing markets successfully requires either a rare (and VERY well-compensated) skill or access to some relevant non-public information. There is a strong prior that you don't have any of those so convincing evidence is required to overcome it."..?
P/S/A: There are single sentences which can create life-changing amounts of difference.