Definitely not. Without knowing who the seller was, it's impossible to say that they knew this was wrong or were being unprofessional. Considering ebay is a clearinghouse of all kinds, its possible that the seller has little or no previous experience with coins. Most of the population does not know how to properly handle coins, so giving a negative for what may have been an honest mistake is overly harsh and completely uncalled for without knowing the facts. A neutral implies a less than positive experience and therefore can hurt. It is possible to ship safely using regular business envelopes. If a coin (flip) is sandwiched in between two larger (near envelope sized) pieces of stiff cardboard and is sent "non-machinable", rarely if ever will problems arise. Not that this is better than a bubble mailer, but is a lower cost and safe method. It's probably safe to assume that after expenses and fees, the seller may have barely broke even on this one. While added care should have been taken, it's not worth knocking the guy over a buck. Chances are the seller is not some horrible uncaring person, so by simply pointing out the risks of their shipping method, you would be doing the right thing. I highly doubt they want unhappy customers, so take the high road on this one by trying to educate and not knock for what may be a simple mistake. You do not have to leave any feedback.
I am not sure ( having not tried) if the envelope with stiffners would qualify for the USPS Delivery Verification. At least at my local PO, they stick them into a piece of cardboard with a 3/8- 1/2 in slot. If they pass through, you have to send by insured , Certified , or Registered( which gives you tracking, but for a higher price). Whereas if you pad a bubble wrap envelope with a small extra piece of bubble, you can make it Parcel Post ( won't go through the slot) which is Verification eligible for much less than the regular services. Jim
This you should leave a negative for because if he regularly ships his coins with excessive amounts of tape onto paper, other potential buyers of his items should know this.
I have received moderately cheap coins from e-bay sellers in packaging that looked like corrugated cardboard that folded over itself and had some sticky that stuck to itself but not to the coin.