1. Auctions: Eyeonwall is correct.
ebay saves that payment information for subequent bids, so presumably sniping services aren't confronted with that barrier. If the very first bid you make with
ebay is through a sniping service, then yes presumably it would be denied. If you want to be shifting bids around between payment sources rather than having a standing payment method, that's on you, not
ebay.
2. Fixed price listings: As a buyer, there's nothing you can do about it; it's the "new abnormal". For sellers, as shown above, you can disable that requirement... which I did on day one. In 25+ years on
ebay, I think I've had maybe 2 nonpaying bidders/buyers. Additionally, I don't mind waiting a week or two to combine purchases.
ebay automagically nags buyers for me anyway, so it's not an impediment. I leave this turned off so that buyers have the option of paying one by one or asking for a combined total.
3. The two areas where
ebay still doesn't have a good solution, not so much due to upfront payment specification, but rather the mechanisms in play:
A. Combining shipping for international orders if you are opted into EIS. Since the lion's share of the shipping costs come from
ebay and those are obfuscated to the seller, there's nothing to combine. Also, sellers explicitly CANNOT combine separate EIS orders into a single shipment, as each one must have a unique code on the address label.
B. Combining best-offer purchases or best offer with normal cart purchases. Cannot currently be done, any combined shipping must be done after the fact and manually refunded. I am constantly asked whether I will combine shipping on best offers. Because it's done after-the-fact, it may actually be a barrier to purchase if the buyer doesn't ask.