I'm presuming this wasn't collection from the seller's home but from a Click & Collect point ?
If 7 days have passed since you were notified it was in store, then you'll lose all buyer protection under eBay's 30 day MBG. The store can either send it back to the seller, give it to charity or even destroy it. You cannot extend that collection time.
If the item is sent back to the seller, they'll need to refund you, and can if they wish to sell to you again, relist it for you to buy the item again from their eBay listing.
@milne_4517