when the store sell to you after May 18 the store will get RCMP for approval. And the government will know you are the new owner. If you plan to sell it in the future without informing the government, they will still assuming you own it. Any crimes potential done by that firearm will come back to you.
However for current non-restricted firearm owners, you can always claim you sold it before May 18. To conclude, it is more a issue for future non restricted trade.