Just picking up on NoVaguy's correct observation about sleeve length, the way to do the measurement is as follows: Start at a point exactly midway across the collar seam at the back (just under the bottom of the collar) and run the tape measure along the shoulder until you come to the seam between the shoulder and sleeve. Then angle the tape down a little and run it from that seam down the sleeve to the end of the cuff. I've done this countless times, and I've noticed that if you aren't very careful in positioning the tape measure at each step, your measurements won't be very repeatable. To locate the exact center of the collar seam at the back, fold the shirt in exactly half and run a pin through it at the collar seam. Then start your measurement from the middle of the pin location each time. Smooth the fabric out as you go, so that you don't underestimate the length because of bunched-up material. I usually do the measurement 2 or 3 times and take the average. Once you know what this measurement should be for you so that the sleeve extends maybe 1/2" past your jacket sleeves, that measurement should apply to all future shirts you buy.
Edit: Oh, BTW, I'd ask the seller to take the sleeve measurement for you so that you know that the shirt will work for you. You should determine your correct sleeve length in advance, and compare what the seller tells you about the shirt for sale. The big problem is when sleeves are too short. There's just next to nothing you can do about them (unless they're like Hilditch & Key shirts with a built-in extra 2 cm. of fabric buried in the cuffs). On the other hand, too-long sleeves are easily shortened (a $10 job) to exactly the right length. I always make sure that RTW shirts I buy sight unseen have sleeves that are a little too long. Then, I have them shortened to the length I want, adding about an extra 1/4" to the right length to handle shrinkage in laundering. I wash my own shirts in cold to lukewarm water and hang them to drip dry--never, ever letting them near a dryer. I've found that doing them this way keeps shrinkage down to about 1/4" or so over many washings--although I hasten to add that the fabric used will have some effect on the amount of shrinkage. Another thing that could be done is to wash the shirt while the sleeves are too long, so that first washing and accompanying shrinkage (which is usually more than that from subsequent washings) are done before the sleeves are shortened to the exact right length. It's truly an inexact science, but that regimen is the best I've been able to come up with.