Gollux Penny

Hey guys, so i decided to try kill Gollux and i've got 118 Gollux Penny and 18 Gollux Coin... Is there any way to exchance the penny into a coin?
and another thing: can i only buy 1 thing in the store? because it dont allow me to buy stuff that i bought before...

No.

Shop's glitched.

If the shop wasn't glitched, technically every 10 blue coins is a key, each key is another run, which gets you 3 green coins (5 if you complete the run, I guess, but you should only be doing that once a day). Right now you can only get 5 keys from the shop before it stops letting you buy them.