Can dealers install purchased parts from online?
 

I'm buying a pair of new shocks for my car off ecs tuning and seeing if they are able to install these so I don't have to buy oem original from the dealer

It's the choice of any shop. Call them and ask.

Yes, we can and have on numerous occasions. Only discalimer is, as you'd expect, there's no warranty coverage from the dealer EXCEPT of course, the installation workmanship.