"Sad fact of life"
This is an unfortunate turn of events, we can safely say without any shadow of doubt that many potential newcomers to Numismatic collecting will start their collection off only to realize later that the coins they bought in good fave were in fact fakes.
There are so many con artists today making a fast buck that the safest way to be sure of buying genuine coins is to do so via reputable vendors.
Being able to spot a fake is not the easiest task, only in educating yourself through reading material and posts will help to minimize the risks and maybe deter future counterfeiters.
Oh i forgot, gut feeling (if in doubt, dont buy) this has helped me on numerous occations.
Concerned collector.
This is probably overpessimistic, though you are right to use the "if in doubt, don't buy" control.
ALL you need to do to avoid
fakes or altered coins, is to avoid
eBay, and buy your coins from trusted retail sources, such as
Forvm here, (or other trusted dealers whom you have bought from e.g. via sixbid or
vcoins).
There is no need at all for any worries about
fakes. If you buy from retail dealers who are well known and have the expertise to authenticate all the coins they sell (NB I am including here the 'ebay arm' of such retail dealers, e.g. when
Forvm or HjBerk sell on
eBay), then you will not buy any
fakes. This is a simple solution that I've hammered away at for years, and I guarantee it works. It worked for me in my initial decade of coin collecting, and it will
work for everyone here. If you follow this guidance then you will never need to worry about sellers such as
numis18 or other
NFSL sellers ever again.
It's in your own
hands. Choose the right venue, e.g.
Forvm, or another trusted retail venue, or choose the worst most unregulated market in the world.