New tall boot help asap!

Ok, so I just bought a brand new pair of field boots on eBay. They are the kind that have the zippers on the back. I've had my foot measured and have tried on several different brands of tall boots, and have been a 7.5 in all of them. When I got my new boots in the mail today, I instantly had to try them on just to make sure...but the zipper wouldn't go up all the way! It was physically impossible and I tried more than once. Does this mean they are somehow the wrong size for me? Or is this a new boot thing that can be fixed? PLEASE help. Unfortunately I don't think the seller accepts returns...is there something I'm missing?

Well depends on how much give you need... mine are always super tight and I cannot get them to go all the way up but once I soak them in warm water then wear them around till they're dry, then condition them once theyre dry they fit perfectly

You could have them stretched at a shoe repair if they aren't too off. Put on a thin boot sock and see how far off they are. Take them and ask, if not can you return them?

If the shoe repair person can't stretch them enough, you can also see if you can add an elastic strip along side the zipper. Not only will this give you more room, but it will take a lot of the stress off the zipper and will be less likely to break. Mant custom and high end boot brands do this now.