I buy alot from QVC in my 4 favorite brands, Denim & Co, Isaac Mizrahi, Belle and Susan Graver. I havent had any issue with sizing in the same garment. I would not want the try before you buy.
There would be so many items that would be ordered just to try on then returned after having been worn by who knows who as they changed their mind about buying.
I'm also thinking of sell outs as people would order just to try on in different sizes and colors. Lets out those of us who want that item and it would not be available to purchase as someone decided they wanted to try it on.
Then the return nightmare at QVC inspecting try on returns, re-packaging, returning to inventory. All that extra labor means is increased prices.
If you want it, buy it. If it doesn't fit, return it. QVC has one of the lowest return shipping fees but read the measurements before buying and save the returns. If it is defective there is no shipping fee. I am definitely not in favor of try it before you buy it.