“Fifteen years teaching experienced qualified full time tutor - Available for all levels Chinese subjects.”
㈠ 构词法不对：fifteen years→fifteen years of /15-year ；all levels→all levels of. ㈡ 名词形式不对：experienced→ experience. ㈢ 标点符号被忽略：full time→full-time.
A qualified full-time tutor with 15 years of teaching experience is available for all levels of Chinese.
“All materials are good quality with latest design and guaranteed.”
㈠ 形容词词组不对：good quality→of good quality. ㈡ 定冠词被忽略：with latest design→with the latest design. ㈢被动语态中的接系动词不在场：guaranteed→are guaranteed.
All materials are of good quality, with the latest design and under permanent guarantee.
“To sell or to market such a high value asset professionally, you should entrust the job to a true professional in order to optimise your property's full potential to secure the highest price and the shortest period of time.”
㈠ high value→highly valued/ highly valuable. ㈡ your property's full potential→the full potential of your property. ㈢ and→at。
此外，㈡前面有“optimise”（尽量提高）这字，“potential ”里面的“full”便多余了，应该除去。同样的道理，下面这句里的 “widely”和“in more than 100 countries”，也互相抵触，要除去其中一个：
“...prepare students for the Royal School of Music Examination which is widely recognised in more than 100 countries.”
改正之后，可以是“which is widely recognised”或“which is recognised in more than 100 countries”.
此外，这句的形容词分句“which...”是非限定的（ non-restrictive adjective clause），其前面要加逗号，把分句和先行词分开，才符合用法：
...prepare students for the Royal School of Music Examination, which is widely recognised
...prepare students for the Royal School of Music Examination, which is recognised in more than 100 countries.