As mentioned above, any shareholder can be a sole representative for the firm. So, yes if you sell your shares and the ownership is now below 50%, you can apply for the sole representative visa. However, as you are required to provide documents from the previous year, you would have to also provide evidence of selling your shares since then with an up to date share register as well.
You should not have a majority stake or otherwise control of the company, directly/indirectly.