I'm still mystified...

Saying "sales tax" is part of this thing is not a good example because you have sales tax with any item purchase you make locally in your own state.

I do think people forget that when they buy a product at MSRP, shipping and transaction fees are included in that price.

That's right. It shouldn't be any different when we are selling. It's a business transaction and the seller should factor PayPal fees into his/her price.
Seeing that "add 3% for PayPal" really irks me.
   But, many times the seller has lowered his price slightly when I've asked. They know the 3% is out of line.

This is what I do. If I find an item that I want, priced right but wants 3% PP fees, I ask what the shipping cost is, then total it up, deduct 3% to 5% and make them the offer. It works just about always.

Some sellers are offering items at very attractive pricing with no additional room for the 3% fee. Budget minded Aphiles are then given the option of paying by check, cashiers check, bank transfer etc... to save another 3% on the purchase which for some is very attractive. I like the choice and opportunity to save even more money. A $5,000 item will add another $150 in paypal fees. That is a lot of money.

No reason for this to upset anyone. I suppose I look at the glass half full, not half empty here. That may be the difference. This is done quite often with credit cards. The vendor often offers a savings to buyers who pay by cash or check. The vendor is simply passing on the savings of avoiding the credit card fee. Same sort of thing. Very common and I like it.

If an item is priced rather high with the only payment method accepted being paypal and the fee paid by the buyer, then you can offer what you feel is a reasonable price. Wether you pay the paypal fee or not is really immaterial as it always comes down to the total cost of a purchase with fees such as shipping and paypal being part of the equation and decision.

Here is a good example of what I am talking about. The paypal fee on this item is over $350! The seller gives you a way to save money if you want and priced the item reasonably. What could ever be wrong with that? It’s all about choice. If you don’t care about paying the fee and want the convenience, then go ahead and pay by paypal. If you want to save money, send a cashier check. All of us get what we want and the seller is being very smart by offering options.

I dont know the seller and just just picked the first new ad I saw listed.

https://www.audiogon.com/listings/lis959j8-dynaudio-c4-signature-platinum-w-crates-full-range