Wow, so many people really don't listen to you all? It's not like you'd be saying something that didn't make sense, so why wouldn't someone listen if the advice sounded remotely reasonable? X_x
Every time I'm in my local Petco/Petsmart/Etc for plants and basic maintenance stuff, I dish out advice if I know something that applies. More often than not the employees actually hand a customer over to me and/or listen themselves. And I may be older than most of the employees, but I'm so baby-faced most people think I'm 14 or 15 (which sucks by the way lol) and they still listen.
The only trouble I've ever had is with the manager (Petco), who is content to blow his stack and yell 'till he turns red in the face on a regular basis because I'm stopping "potential" sales. But the angry little man hardly phases me and I suspect yelling that you're only about sales and not the animal's welfare in a pet store in front of customers is eventually going to bite him in the arse.