I am looking at replacing my chain rings.
I currently have FSA SL-K 50/39 crankset.

Am I correct in saying that as long as the chainrings are 50/39 and the correct size and bolt location then the chain rings are universal?

Reason being...I've seen FSA chain rings and Ultegra chain rings on ebay....prices are about $10 less for the ultegras.

check the bolt center diameter (BCD...google sheldon brown & BCD if you want to learn what it is/how to calculate/etc.), but generally these days I think if it's road (not compact) & not Campy, it's 130 BCD. That said, with the tooth counts you have, you could also have a 'compact' crank, which would be 110 BCD. I'm probably making this sound more complicated than it is...it's simple.

OP is mistaken about one of his rings. He probably has 50/34 compact with 110BCD but he might have 53/39 standard with 130BCD. You need to be very sure about this, obviously.

rings aren't quite universal, but FSA, Shimano, SRAM (except for X-Glide), Praxis and Rotor will all fit your compact or standard crankset. Shimano and Praxis have significantly better ramp & pin design than FSA, at least the FSA I've used. so I think the Ultegra rings are a good option.

I have a 105 crank that is 39/50..sort of a nice touring/commuting range & this one happens to be 130bcd, so such a combination does exist....and, yep, it came from a factory that way.

FSA makes a 50T, 130 BCD ring, so the OP may be correct.

Its a 53/39 130BCD.