Beth,
It's always fun to read about someone else's clutziness, then I don't feel so bad if I do something clutzy!
Re: the salesman (sales "boy"?): What nerve of him to imply that you shouldn't be buying weights you can't lift. And how ignorant! You may not be able to lift 40# or even 20# in a regular carrying position, but you might be able to do a 1-arm lat row with the weight.