We buy a coin under three conditions:

1. We like a coin.
   
2. We trust seller.
   
3. We have enough money.
   

Do we really need a coin? Let’s confess frankly: we decide that on place of purchase.
Thus, a seller must:

1. Be sure that we have enough money.
   
2. Make us trusting to (s)he.
   
3. Make a coin attractive.
   

Usually we are going to the shop or street market and browsing e-auctions, having some money to buy.
Usually we trust to seller, being based on his reputation. More precisely, we think that. To learn how to do a clever face please read Dale Carnegie. To learn about hiding disinformations within real information please see movies on spy games.
Below I list several tricks of sellers, appointed for raising the coin attractiveness.

Polishing

The use of common means for precious metals cleaning is very effective way to raise an attractiveness. That is available for coins of VG-F-VF-XF conservation. Certainly, AU and Unc coins will be spoiled by this method, but other coins are losing ugly spots and deep scratches. Sometimes an ugly XF coin can be turned into more valuable VF+ item.

Artificial patinization

This is very popular method of coin value rising. The most of collectors prefer the patinated coins as more esthetic. Besides, the patina appearance is considered as an attribute of good storage of a genuine coin. We can roughly divide the method of artificial patinization onto three types:

Home-made oxigenization

You should not be a chemist to do that. You should come into your bathroom and find a liquid for the lime and rusting removal.
The cleaners, containing ant acid or alkali, are not suitable. Take a cleaner with hydrochloric or phosphoric acid and put gloves on your hands. The home acid cleaners are viscous, dense. They will give you dark black oxide or so called rainbow patina on a silver coin.
What kind of patina you will obtain is not possible to guess due to the great difference in the coin alloy components and because any coin has a microparticles of other metals on the coin surface, which give indeed a multicolor instead pure silver patina (it’s black). Surely, using this method you will undoubtedly spoil an AU and Unc coin. Generally speaking, only AU and Ucs coin having a rainbow patina are really deserving an especial attention as genuine and well conserved.

Non-oxigen patinization

Many people know that good black patina can be created by using photographic neutral fixative (sodium sulphite), which acts in this case in the same way as in photographic process. It contacts with insufficiently oxidized molecules of silver and forms a sustainable compound of the black color. It’s sustainable in chemical sense, not in physical one. A sulphite patina can be erased with tissue or washed out with soap. Many couterfeited coins, which we can find at Internet, contained a bit of silver, cast of aluminum or zinc, can be oxigenated at home in order to make us happy bidders.

Pseudopatinization

A mix of black ink and silicate glue is a good imitation of black patina. I have dozen pieces of eight smeared with a similar mix. Naturally, I washed these coins.

Varnishing

It’s very effective imitation of fine conservation of a beaten or scratched coin from bronze or brass. Cleanly washed coin covered with matte varnish can be recognized up to 1-2 grades higher.

Glue covering

Some collectors cover their coins with varnish for better conserving. The others use for this goal different glues of polyamide or of acrylic base. Such glues remain elastic after hardening. They fill all scratches and hollows, and the thin slice of glue is often indistinguishable from the bronze patina. I have 1 kroon of Estonia, on which I found scratches and edge beats under glue slice only with 10x zoom.

Dirting

Now I beg you pardon for physiological details, but you know that some persons have black strips under their fingernails. Exactly such phenomenon we can see on coins. Don’t believe that it’s patina! More often it is just dirt. I beg you pardon once again, but please don't hesitate to spit onto a coin and you can see what you keep in your hands. The most of street sellers never wash their coins, and they do that consciously for hiding defects.

Indeed, if you are an experienced coin buyer, you will never be tricked. I hope.