Shipping is something painful. Try looking up on the USPS shipping calculator what it costs. It's educational. I try to keep shipping costs down but decent boxes and packing materials are not free, and that trip over to the PO is not fress e. Only good thing at the PO is that I can generally use the self-serve system to avoid spending 20-30 minutes in line.
I agree with all you say, but sadly it doesn't help.
As for shipping, I pretty much quit selling as the shipping cost were too much and buyers complained. Then my wife informed me we have a Stamps.com account....which she uses for stamps.
I've found I can ship UPS, Fed-X or USPS via this account for around half of what it cost to walk in a carriers office and especially the boutique mailing/shipping stores located everywhere. The last two boxes would have been $20-25 using the the normal shipping way, but Stamps.com was $10 on one and $12 on the other. That is a big savings.
Now, you do have to ship enough to make it worth the $20 monthly fee to have this account, but if you ship very much it pays for itself over and over.
I've also learned that a company called "Pirate Ship" is good for larger boxes like car parts, etc.
I've had an EB account since 1998, back when it was a great place to sell. But, the last time I sold (few years ago) they asked for my SS# and said they had to notify the IRS of any combined sales over $600 a year. Maybe not a big deal and maybe not their fault, but it really chaps my *ss they want/have to do that, so I quit selling there.
Yard sale is hit or miss and a much smaller audience, so I think I'll have to bite the bullet and start selling my excess trains on EB again.