Canon 85mm lens good

Hey guys, i am about to purchase some 85mm lens by canon. i was looking at the 1.8f. however, as i made the search, i noticed there are two 1.8 options. one is $349 and the other is 365. they have different serial numbers...thats all i really see... am i missing something? 

lens owned: 

-canon 50mm 1.8 
-canon 50mm 1.4 
-canon 60mm (macro lens which are worthless to me...) 
-canon 135mm soft lens 
-canon 18-55mm 

i want to upgrade from the 50mm to the 85mm. I've been shooting with those for 4 years. time to move on:)
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All decent photographic equipment; cameras & lenses have their "own" individual serial number. The serial number is used for identifying purposes. 1. Such as repair: the serial number denotes which lens is YOURS because you listed it on the letter you sent with the lens. 2. The serial number appears (or should appear) on your list of photo equipment you took out insurance on! 
Now the serial number difference has been solved for you, the next important thing especilly for a buyer is the quality rating of the lens. When you purchase from people like B&H Photo or Adorama camera shops in New York they give rating to each used item as to its condition. So what you are missing to use your words is the condition of each lens and the fact that retails can & do set their own prices. Now the next thing to consider is the plac seling the item: Is their rating good as a Amazon seller? I am with contirbutor "retiredPhil". The $349 lens is said to be "new" and hasa USA & Canada warranty (as long as you live in one of these places) I too would make this my choice! 
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The most important question is do you need a 85 mm lens? 

Do you have a full frame Canon camera, a 6D, 5D or 1D camera? 

Do you know what a new Canon EF 85 mm f/1.8 costs new? $420. The two links seem to list two different new price lists for the same lens. 

Of course the lenses have different serial numbers. Each lens has a unique serial number. 

Different sellers from time to time sell products for prices less than the manufacturer's suggested retail price (MSRP) 

I would be more concerned that one or both of those lenses are grey market or refurbished ... something that should be stated in the Amazon adverts. 
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If you do buy this lens, a minor $16 difference in cost is really not much of a big deal

There's no such thing as a Canon Mark 3. 
Two of the lenses that you have are not even compatible with any of Canon's cameras that have Mark 3 versions. (1D Mark 3, 1DS Mark 3, 5D Mark 3) so it's really not clear what you own. 

And going from 50mm to 85mm is not an upgrade. It's just a different focal length. 

But yeah, they're the same lens.

Those two lenses are the same lens - two sellers, two slightly different descriptions of the same lens, and two different prices. Amazon is a market place, everyone selling stuff on Amazon has their own prices. 

Every single product in the entire world, that has serial numbers, has a different serial number for each item made. Perhaps you live on a different planet? 
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What on earth makes you think an 85mm is an "upgrade" to a 50mm lens? These lenses have completely different focal lengths, one is not better than the other! It would seem you are clueless. And by the way there is no such camera as a Mark 3. You missed out the model number - is it a 1D, 5D, 6D? 

"I've been shooting with those for 4 years. time to move on" - I have lenses which I have owned for 32 years, and which I still use. Lenses can last a lifetime. It's not "time to move on". It's time to learn what your lenses are for, and the difference between different focal lengths, and their uses. 

"lets not be idiots here" - seriously!?
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