XtriC

+10 @luizragna thank you for your kind help

Try now:
I changed the coin to item ID 26080
NPC Trader:
prontera,136,217,4 script Coin Trader 4_M_MANAGER,{ .@npc$ = "[Coin Trader]"; //NPC Name mes .@npc$; mes "Hello. I'm the Coin Trader"; mes "I can trade Zeny for Coin and vice versa"; mes "(1 Coin = 10.000.000z)"; mes "What you want trade?"; next; menu "Zeny for Coins",zc,"Coins for Zeny",cz,"Close",cl; cl: close; zc: mes .@npc$; mes "How many Coins do you want purchase?"; mes "(1 Coin = 10.000.000z)"; input @zc; @qnt = @zc*10000000; if (Zeny - @qnt < 0){ mes "Do you not have zeny"; close; } getitem 26080,@zc; set Zeny,Zeny - @qnt; mes " "; mes "Thank You!"; mes "(Now do you have "+countitem(26080)+" Coin(s) )"; close; cz: mes .@npc$; mes "How many Coins you want sell?"; mes "Do you have "+countitem(26080)+" Coin(s)"; mes "(1 Coin = 10.000.000z)"; input @cz; @qnt = @cz*10000000; if (countitem(26080) - @cz < 0) { mes "Do you not have sufficient Coins"; close; } if (Zeny + @qnt > 2000000000){ mes "Do you can't stay more than 2b Zeny"; close; } delitem 26080,@zc; set Zeny,Zeny + @qnt; mes " "; mes "Thank You!"; mes "(Now do you have "+countitem(26080)+" Coin(s) )"; close; }  
Coin Shop:
prontera,141,217,4 script Coin Shop 4_F_TELEPORTER,{ mes "[Coin Shop]"; mes "Hello. Why do you want buy with yours Coins?"; mes "(Do you have "+countitem(26080)+" Coin(s) )"; menu "Red Potion (3 Coins)",rp,"Knife[4] (7 Coins)",kn,"Cancell",cl; cl: close; rp: if (countitem(26080) - 3 < 0) { mes "Do you not have sufficient Coins"; close; } delitem 26080,3; getitem 501,1; mes " "; mes "Thank You!"; mes "(Now do you have "+countitem(26080)+" Coin(s) )"; close; kn: if (countitem(26080) - 7 < 0) { mes "Do you not have sufficient Coins"; close; } delitem 26080,7; getitem 1202,1; mes " "; mes "Thank You!"; mes "(Now do you have "+countitem(26080)+" Coin(s) )"; close; }  

I maked with variable.
 
Trader NPC:
prontera,136,217,4 script Coin Trader 4_M_MANAGER,{ .@npc$ = "[Coin Trader]"; //NPC Name mes .@npc$; mes "Hello. I'm the Coin Trader"; mes "I can trade Zeny for Coin and vice versa"; mes "(1 Coin = 10.000.000z)"; mes "What you want trade?"; next; menu "Zeny for Coins",zc,"Coins for Zeny",cz,"Close",cl; cl: close; zc: mes .@npc$; mes "How many Coins do you want purchase"; mes "(1 Coin = 10.000.000z)"; input @zc; @qnt = @zc*10000000; if (Zeny - @qnt < 0){ mes "Do you not have zeny"; close; } set SCoin,SCoin + @zc; set Zeny,Zeny - @qnt; mes " "; mes "Thank You!"; mes "(Now do you have "+SCoin+" Coin(s) )"; close; cz: mes .@npc$; mes "How many Coins you want sell?"; mes "Do you have "+SCoin+" Coin(s)"; mes "(1 Coin = 10.000.000z)"; input @cz; @qnt = @cz*10000000; if (SCoin - @cz < 0) { mes "Do you not have sufficient Coins"; close; } if (Zeny + @qnt > 2000000000){ mes "Do you can't stay more than 2b Zeny"; close; } set SCoin,SCoin - @cz; set Zeny,Zeny + @qnt; mes " "; mes "Thank You!"; mes "(Now do you have "+SCoin+" Coin(s) )"; close; }  
Coin Shop:
prontera,141,217,4 script Coin Shop 4_F_TELEPORTER,{ mes "[Coin Shop]"; mes "Hello. Why do you want buy with yours Coins?"; mes "(Do you have "+SCoin+" Coin(s) )"; menu "Red Potion (3 Coins)",rp,"Knife[4] (7 Coins)",kn,"Cancell",cl; cl: close; rp: if (SCoin - 3 < 0) { mes "Do you not have sufficient Coins"; close; } SCoin = SCoin - 3; getitem 501,1; mes " "; mes "Thank You!"; mes "(Now do you have "+SCoin+" Coin(s) )"; close; kn: if (SCoin - 7 < 0) { mes "Do you not have sufficient Coins"; close; } SCoin = SCoin - 7; getitem 1202,1; mes " "; mes "Thank You!"; mes "(Now do you have "+SCoin+" Coin(s) )"; close; }  

OK, I'll try to do it for you.
You want the currency is for a player or account?