You've done things the wrong way around, in that you needed to deal with the first purchase properly before moving on to the next.
So in this case, you should have requested a refund and waited till you got it. Actually, if the seller was telling you the item was defective, he should have issued a refund on the spot if the plan was not to send it. I am not sure why this did not happen.
However, the seller cannot just send you 'another product' unless it is another one of exactly what you bought.
As it stands though, when a seller sells a thing, their job, as far as ebay is concerned, is to get the product to you by the advertised delivery dates. All your private messaging is not in the ebay system as such, not on their radar. It sounds to me as if they either fixed the item they sold to you or else managed to find another one and got it posted to you within 2 days. If that is the case, you are likely to receive it within the advertised time.
How to geta reimbursement? I am not sure you can as it doesn't sound as if any official ebay claim was ever opened. Plus the seller would not have known you bought another one so got it off to you asap.
However, you are eligible for a reimbursement if
-the item arrives and it is not EXACTLY what you bought or as described in the ad
-the item arrives and is faulty in any way whatsoever
If that happens, go straight into a proper ebay claim for a faulty product. You get 30 days after arrival of the item. That's all. The seller would then need to issue you with a postage return label at their own cost and you would then return it for a refund in full.
