Tri bike needed $500-600

I made a mistake in my description I am looking for a size 52 or 53.

... or a 54, or a small, or a medium, or a 49, or a 56... yes, all those can be correct. There's NO standard how a bike is denominated nor how it's measured, so the only way would be to mention brand, model and model size denomination. Otherwise, you're WTB is pretty impossible to answer....

my two pennies says try CL that way you can at least go and see it, ride ect.

Here's the problem... you're looking at bikes that are widely apart in terms of fit. A Medium Trek SC2.5 is somewhere between a 51 and a 54 Cervelo P2C, and somewhere between a medium and a large QR Kilo... And even that doesn't mean they are the right size for you... You can take 4 people, all 5'9" and one may fit the QR, one may fit the Cervelo and one may fit the Trek, and the fourth, really doesn't fit any of them... But the chance of taking the person that fits well on the QR and fit them well on a Cervelo is kind of slim... just two very different fits....

For that kind of money, you are MUCH better off buying a decent road bike. You can get a fairly good, used, road bike for $600 that will be a MAJOR improvement over your MTB, but you will struggle to find a decent tri bike for that price... it's as simple as #'s sold... there's many, many more road bikes out there then tri bikes... which drives down the cost of a new, and further drives down the price of used...