Curious if you've met the OP in real life to be so sure? For all we know the 54 isn't a correct fit for him to start with.

If you hold your hand up and measure 5cm with your fingers it's just less than 2 inches. That's the difference between the seat tube height of a 49 and 54cm frame. Depending on how high the rider puts their seat in relation to the bars, how long the seat post is, the angle of the TT, length of crank arms etc... that may make a difference - or not.

As it is, seeing as it's cheap and the OP wants the components, he could buy the bike even if it didn't eventually fit, swap all the components onto his current ride and probably still sell the other bike at a profit.

And along the way see if the second bike actually fits.