I know how to make all the events concerning the coins and if the player has enough coins to buy the item, but I don't know what the best way is. Is it like to animate it or to make it easier change the whole object itself? Thanks!
It really depends on your visual style for your game, you could do it either way,
In my space shooter (in my sig) my in game shop currently just uses sprites that get color changed to dull color when item is bought.. it fits the "theme" but in my final version I will probably use a nice looking sprite but still dull it out when inactivated.
Animation in Construct2 is pretty straight forward. You either drop in a sprite with multiple frames or you make your own via the in game editor. Either way you control the start/stop/frame/speed etc via event actions,
It shouldn't take you too long to get your head around it.
A simple exercise is open sprite editor (double click layout and select sprite) and color fill the image. Now find the small frames window that opens when editor is open and right click beside "frame 1" and choose "add frame", now color fill this on something different. Repeat until you have about 5 frames of different colors. You can use the animations window (that also pops up when editor open) to save this to a anim file you can call via event if you like. Now with a frame of the sprite selected look at the left side properties area and stop it from playing automatically. These properties control how sprites frames are "stepped through".. aka animated. Now place your cool sprite on the layout and open your event sheet. Add event On start of layout then select your sprite and choose start anim frame 0 and go run program.. voila you made an animated sprite. The principles are the same for any 2d animation outside Spriter type bone animation.