Replacing Tiagra 9-speed shifter with 9-speed 105 or Sora shifters
You can use either the Sora or 105 shifters (all the cable pulls will match). Sora 9 speed has the proper ergonomics (instead of the silly button that it used to use on 8 speed), so its a good shifter. 

As for if you will notice any performance difference, both should shift fine (if the 105's are indeed in good condition) [if you're racing, maybe you'll notice a difference], but the ergonomics may be very slightly different between the tiagra-105-sora levers (but you'll get used to it) [it wouldn't surprise me if the current Sora levers are a slight modification of the 9 speed Tiagra levers due to trickle-down]. 

I'd probably suck it up and buy the Sora levers personally - brifters are too finicky/fragile in crashes for me to be comfortable buying used ones off eBay unless the price is very good. 

My guess is the bike shop guy wants you to go 10 speed which will involve replacing a whole bunch of stuff, hence the upturned nose. Or general cyclist snobbery.