If the box had come with the tape seals intact, I would have given the seller positive feedback and moved on. If the seals had been cut and the bag inside was a standard issue unopened Lego parts bag and all of the parts were in the bag, I would have still given the seller a positive feedback and moved on. Because it would be plausible that the seller opened the box to make sure everything was inside.
Would you really? You decided a 25% refund was called for because of the tape seals? I collect a lot of stuff - some carded and some loose. If I wanted this item sealed and it was opened, I'd have asked for a huge discount or sent it back. I couldn't display it the way I intended, so I would be very dissatisfied. The seller was totally wrong here from that angle. BUT, if I planned to open it anyway, I would have just let something like this slide. I might even warn the guy to use better descriptions, but I wouldn't have sought out a discount or taken it any further.
I have actually been fairly lucky with Ebay throughout my collecting career, but I hear stories like this all the time. Big companies like Target can weather a dissatisfied customer here or there. Maybe I'm just over sensitive to smaller sellers since Ebay already dicks them over for 10% of final value and shipping fees on top of paypal fees. Glad you at least got the item you wanted.