Overall, on eBay you are the seller and can ship your orders using whatever shipping method you want.
Mostly it comes down to how valuable the goods you are selling are and whether you can tolerate problems that you may have to deal with down the line. Generally speaking, the cheaper the packaging and shipping options you choose, the more problems post-sale that you will have to deal with. Of course, on the other end you also probably don't want to ship everything via FedEx Overnight in foam-lined safe deposit boxes due to cost, so it is a spectrum.
Note that shipping anything on eBay without a tracking # will open you up for INR (Item Not Received) claims as there is no way to prove the order was delivered. So, in that sense, you would be placing a lot of trust in your buyers.
If you ever want to be a Top Rated Seller and get associated 10% off final value fees I think you have to use package tracking.
If the cards are valuable and condition is important, you also have to worry about folding/bending of letter sized envelopes as those are machine processed.
