There are three groups of people that can buy Diamond Anvils to regularly upgrade their equipment:
1.) RMT (pay2win)
2.) People who spam leti cubes and don’t sell the diamonds (pay2win)
3.) People who farm silver all day every day and buy them off the MP (does this exist?)
The rest of us either sell Diamonds when we happen to get them, use the ones to get a certain weapon to +16 or higher someone higher (if you are lucky), or simply don’t use them.