These are a problem, aren't they? When I was in retail we'd have to eat the cost of most returned items unless they were defective. REI gets returns like this all the time, and they end up in their quarterly garage sales. BPL has some items like this, used or opened packages, that they are selling at some fraction of full price.
You could say "no", and the customer might or might not buy the pyramid, and would eBay the tarp or find some other way of selling (or using) the item. The customer doesn't seem to have an expectation of you taking it back, but then there was no harm in asking.
But since this is a "never used" item, however, the seam sealing could be a bonus for another customer. If it's not too hard to modify your website, you could probably even put this one on your tarp page and sell it quickly for full price to somebody who doesn't want to pay $6 for seam sealer and spent the time and effort to seal it.
It is an interesting question, since there don't seem to be any customer or general-audience repercussions to your decision, and possibly no loss to you involved either. Just one of those "how do I feel today" decisions.
Thanks for helping me avoid work this morning :)