Maybe the shop is trying to rip you off, but unlikely. Lots of parts are hard to source right now, and it is possible the suppliers they deal with don't have the exact parts you need. But if all they are doing is 'trying to sell you stuff' then that isn't ripping you off. If they promise a certain product or service at a certain price, then deliver a different (inferior) product or service for the same price, or try to charge you more than they initially quoted for the product or service, THAT is likely to be trying to rip you off.
Saying they can't get the part you are looking for? Not a rip off. Trying to sell you something more expensive - also not a rip off. You need to grow your skin thicker if you plan to keep existing on this planet.
Also, if you can get the parts you need for cheaper than they can sell them, then go ahead and buy them - chainrings aren't that difficult to install, nor are cassettes, nor are chains. Do the work yourself and you can be sure nobody is trying to rip you off.