You bought from them, you're giving first refusal to them. I can't see a huge issue. People's tastes and circumstances change, they really wouldn't have much of a case to Blackball you just for wanting to downsize your collection.
I suppose it depends slightly on the models and how long you've owned them - and particularly if you've bought anything else since. Meaning, it's far more logical for you to want to sell some models that have been superceded by new shiny things.