At first I thought that even if you had been a member of ebay for a few years, this may have been your first problem so maybe you just got caught out.
But I see from the feedback you have left in the past that you have had a few other negative experiences.
When that sort of thing happens, it pays to actually go onto the site of wherever you bought from and read up about terms & conditions and what you can do if there is a problem.
Many 'real' stores will do an exchange for you but that's different as you are there in person.
If you try to do things behind the scenes on ebay, as you did, then things can go wrong.
I know ebay makes it sound as if you should work it out with the seller privately but quite frankly, you are better to just do it all through official ebay channels.
You may be in trouble now because it may be more than 30 days since you received your microwave. If so, then you're out of luck.
If it is less than 30 days, hop to it and open an ebay claim for item not as described or faulty or whatever the wording is.
I'm not sure how it will go, but if you have the tracking number, get a screenshot of the proof of delivery etc which you can use later if you need to.
If you're out of time for ebay and paid via paypal, try opening a claim that way instead. (But never both at the same time)
I can't guarantee either way will work now but I think they are your best bet.
And as jelly bird said, don't use ebay for electrical type purchases in the future.
