Just let off with a warning.

I would offer to put a barrel or can there, inside preferably if you can collect it during business hours. Get you some digital scales and offer to pay them for their trash, I pay $.05 lb for the things you listed from computer repair shops, they call me when their bin is full.

This happened to me a couple times about 15 years ago. Get some bussiness cards and get general liability insurance. $1,000,000 coverage will cost you approx $100.00 month and will cover you anywhere you go. This will also reasure business owners that you are proffesional and not a liability ( That is their biggest concerns ). having the cards and insurance will also up your chances by ten fold getting new clients. Then when you get permission from a company just have them sign the back of one of your business cards.

I know having a monthly expense is a burden and 99% of the people wont do it but this is a start to becoming proffesional and will put you ahead of the 99%. I guarentee that if you had insurance it will pay for itself. It will also start you in the direction of getting out of dumpster diving and getting a small shop to work out of and doing pick ups from the companies you used to dumster dive from.

Also get to know the local laws in your area, dumpster diving is legal in many areas. In the areas where it is legal its considered abandoned property in the dumpster. Having said that arguing with the local law enforcement person is not a great idea. They may consider all scrapers as borderline thieves instead of independent business persons.

I like the suggestions you have gotten so far. I do think you will get shot down more often than not but success is hard work and requires lots of trying. I like the business card idea a lot. As a former professional salesman I can tell you that the very best get told no on the first meeting 9 times out of TEN.

I will share a little sales call advise, here is what was referred to as the four rights of the customer:
1. My name is
2. My company's name is
3. What business my company is in
4. Why I am here today

You would be shocked at the number of "professional" sales people who fail to let the customer know some or any of these. Remember your potential customer's time should be treated as if it is valuable, failure is insulting to him/her. There's lots more to the scraping business than I ever though. Mike.