If I were you, I would wait:
Here are things in your favor:
Good luck!
You can find well centered ones and Mint for cheaper than 20€. Put a notification on ebay with - Shining Legends Mew mint -“psa” - and once or twice a day check it.
Also some people sell as lots…you can get and then resell the other cards, maybe you can even get it for 0€.
Wait for the right opportunity
Shining Mew is quiet playable in decks aswell, thus it keeping value to some degree
Of course it affects value. I’d personally just pay the extra 5 euros.
I don’t see these ever going much less than 20 euros/dollars.
for $4, wouldn’t you trade it for more time with friends/family and time to do other stuff? Time is money. What value do you put on your time?
Is all this over a few dollars? Cmon guys…
It’s only a waste of money if you don’t consider your time worth anything 
I too have paid more for certain cards than i could have paid if i spent some time lurking for deals, simply for the convenience, or buying from trusted sellers where i know “mint” actually means mint.
Newer or mass produced cards don’t take that much time/effort to buy or find better deals.
Rare/mint old cards from different sets are a different beast, though, so finding/buying them can take hours, days, or even years! I spend hours a day on lookout for one certain non-popular-but-rare-as-f card 
It’s not available on eBay right now, but if it does appear, a lucky buyer will get it within minutes no doubt. One time this certain item did appear and then sold out instantly. I missed it because I was in the shower, dammit.
The reason why they’re priced high is because they’re very playable, as much as collectors would like to think the world revolves around us, it doesn’t. Players take priority. I opened a ton of SL and about 30% of the Ultra Rares/Secret Rares were badly centered.
Sell the card now that it’s price highly…played wont give a #### that it’s off centered.
Some of you haven’t bothered reading the OP. He wants to buy a Shining Mew, but he’s being cheap… He’s asking if buying an off centered copy like the one pictured above is worth it or will it affect value when it’s time to resell it.
I like off-center cards and misprints, and unless you’re getting it graded you have nothing to worry about.
It’s not all about you though. We all appreciate the value of money.
I think the difference here is most of us believe in fair and equal deals.
There’s another side to your getting a great deal and saving a lot of money. It means somebody else got a bad deal and you cost them a lot of money. How is that fair or do you just figure, “screw em”? How does that help the hobby we all love?
I think you just completely missed gary’s point…
I’m not sure I follow you here @KingPokemon. I think I normally side with you on most things, but here I just can’t seem to find your point.
I am assuming when @carrot made his deal nowhere was a gun to the sellers head or any coercion involved. If I am wrong in making that assumption, then I completely retract my statement. But I can only assume it was won through an auction or buy it now/best offer situation. The fact is that sometimes sellers need to move things quickly. Need or perceived need is expensive. If you ever need to sell, expect to get under market value. If you ever need to buy something, expect to pay over market value. If this buyer got a great deal because anyone who owned one was willing to sell at that price I don’t see anything wrong with that. On such a cheap card we are literally talking about a few dollars how was anyone screwed on either side?
So you’re saying his card was done in an auction? Or best offer? If so, I missed that point. Of course if you win an auction the seller and buyer should honor the bid.
To be clear, if you make a deal that both sides agree on then that should end it. I didn’t see that in carrots case but you say it was there.
Anyway, I’m speaking more generally. His case was pretty incidental. You, more than most people, know the incessant low ballers only waste time and hurt the hobby. Time is an important commodity so don’t mess with mine. Hobby health comes in a strong second.
That’s very generous and kind of you, well done.
Unrelated note though but sometimes when I buy cards, sometimes sellers give feebies (mint promo japanese copies) which I didn’t ask for, but it was nice and gave me an incentive to complete that promo set.