The term "Copying" is appropriate here.. in the same way that "Sensations" copied "xCite". Burger King copied McDonalds, Warner Brother copied Disney, Deep Space Nine copied Babylon 5.. etc.
Unfortunately, socially, the term "copy" has a lot of connotations due to the naming of the permissions ("duplicate" would have been better), and the unfortunate choice of name of a certain libSL experiment gone wrong. Previous to that libSL experiment, "using an unethical means to duplicate a model, circumventing the permissions system" was called "ripping".
Depending on how close your "Copy" is, you may or may not be guilty of infringing on the other person's intellectual property rights... even if you made it all yourself and regardless of textures. (how many companies have taken eBay to court over "Replicas"?). That's not the same as theft, but in the social sense of SL, it is.
My advice is that you simply use the other boots "as a guild".. and instead of copying them as close as possible, go back to your real-life boots. Take some pictures of them at various angles, look at the boots closely, and focus your design more on replicating the real boots (or "Source"

.. rather than replicating precisely how one seller made his(hers?).
Don't simply "rip" the model (or copy it very closely), and retexture it..
and please don't try to "make just enough changes that you won't get in trouble".. everyone can totally see through that trick, and you'll be branded as a theif. Make design decisions that are clearly your own.. try and make improvements.. Add features, use fewer prims, see if you can make your boots "better" or "more realistic" than the ones you bought....
And then you won't be guilty of anything more than copying the boot design from RL, and taking some building cues from another maker's attempt.
This is your chance to make a choice, on how you want people to see you and your work from now on. Choose the high road. It's more work, but it's worth it.