Oh OP, if you knew the amount of things that go on you'd be clutching your pears even more than you already are 
As others have pointed out, no we aren't going to take a second set of clothes to change into at the end of the day, sorry. And yes, if we have a coat or jacket to hand we'll normally throw it on to avoid this but it depends on if you take one - I don't take a coat/jacket unless it's absolutely freezing and/or raining. Even then our store is normally really warm, so I'm not going to do my shopping feeling like I'm in the Sahara just to stop you having to go to all the effort of asking if we're on shift.
I no longer work in the food side of retail and I'll tell you that all our stock to be reduced comes off the shelves and goes back to the warehouse about a fortnight before head office sends down the budget for us to reduce it. We are all paid barely anything and living paycheck to paycheck, we keep an eye on what's being reduced.
Also, our managers don't allow us to hold items in the warehouse until we've finished work/are on a break so they make us go and buy them immediately even when on shift 
(and yes, I am aware that's against the rules but you try telling our current store manager that).
I do have some sympathy with you in that I hate that when there's a large quantity of one thing that's reduced someone sweeps in and buys them all leaving none for anybody else. That's grabby in my opinion and it wouldn't have killed her to have only taken a reasonable amount of avocados but, I can assure you, if she hadn't another customer would have done exactly the same thing.
People aren't being rude to you because they think you were BU to expect her to share the avocados, people are being rude because of the tone you use. It's quite clear that you believe us little old retail workers to be below you (you'd be horrified with me because my chain isn't even an upmarket one - the horror!) and that you expect them to inconvenience themselves by changing just to suit you. And then you made things infinitely worse for yourself by saying you refuse to be told you were being unreasonable.
To clarify the point others have made
- YWNBU to at first assume she was working and that the avocados were up for grabs
- YWBVVVU to just reach in and try and grab them without asking first (it would have been a polite thing to do even if she had been working, it's a novel idea being polite even to the retail workers but we are SO much nicer to the ones that are, go on, give it a shot and see. YWBU not to apologise and YABU to expect us to lug separate clothing to shop in once we've finished our shift.
HTH and have one of these too 