You mean they taped the poster?

Sarcasm is the body’s natural defense against stupidity.

If they just applied some light tape to the back of the poster I wouldn't care much, especially if the poster already had various marks on it from similar adhesions in the past, as many of them do.  On the other hand, I don't see how the poster can unroll once it's in the tube anyway so I can't see how the tape would be applied for that purpose.  I imagine the tape had been applied previously when the poster was in storage on the seller's premises.

This is absolutely a bad thing. If the poster was not described as having tape on it in the listing, I would most certainly return it. Tape can knock a poster that was in Near Mint or Excellent condition down a couple grades to Fine or Very Fine. I know it seems like a small deal to those not familiar with the hobby, but tape is considered a significant flaw, especially if it's regular tape and not archival tape. The value of the poster is affected by this. I've run into this a few times before on eBay. Some eBay dealers are not familiar with the hobby and are only concerned about making a quick buck. I would return it unless it was stated in the description.