APS will certify prior to shipping if the buyer desires. However, you are still required to pay for the stamp and the certification fee up front.
Yes, but a major advantage that APS offers: The certs are a flat $25, not tied to catalog value, so depending on what it is you're wanting to expertize, the upfront (and total) cost for the expertizing could be a huge savings over other expertizers.
It has been my experience that virtually all sellers on eBay
You must be limiting yourself to a small subsection of eBay
sellers, as this is not my experience at all. It's all over the map as to what eBay
sellers will allow for extension. Also, you are relying upon their word (and the PayPal 180-day SNAD claim window), and that's just recouping the cost of the item itself. Neither PayPal or eBay
will force the seller to reimburse the expertization costs as those are external to the original transaction amount.
Also, the biggest problems with extension from any source, whether eBay
seller, show dealer, or traditional auction house:
1. Who pays for the cert if it is a negative opinion? You would think the seller does, but this is not universal.
2. Who pays in the event of a declined opinion, which is technically neither positive nor negative. There is most definitely no standard treatment of this scenario, but I've seen enough declined opinions to know that it happens. From a buyer's perspective, a declined opinion is no better than a negative opinion and thus the seller should pay for the cert, but you need to explicitly ask about this scenario.