There aren't that complicated internally, but you can't take them apart. Used injectors on ebay are hit or miss. It all depends on how they were cleaned and how they were stored.
In my case I purchased a set on ebay. When I opened the package I could smell the stale fuel in them. Installed them and it wouldn't even run. Complained to the company and they sent me a second set. Of course they were in the same condition. I sent the set to be cleaned and all but one was stuck. They were able to clean them all up and get them working perfectly. IIRC the injectors were $40 and the cleaning was $60. So for $100 I got a like new set, which is still a lot cheaper than other places.