Problems of English grammar

1. This is called relative clause. The clause 'charging a higher price' is used to describe 'any seller'. In this case, 'is' can be omitted.
2. As usual, this will happen in the structure 'main verb + verb+ing'. For instance, 'stop eating'. There are words which can be followed by a verb+ing, such as stop, start, regret, admit, deny, (cannot) help, enjoy, like, love...

....any seller charging a higher price would...

1.點解Seller後面唔洗加is之後ing??呢個是甚麼的grammar?

從文法上來講, 這句是用了分詞短語作定語. (charging 是 charge 的現在分詞形式 present participial)
charging a higher price 在這裏做為定語,修飾/限定 前面的 seller, 可翻譯為:
...任何索取更高價格的銷售者將會...

又例如: The girl singing in the hall is my sister.


2.有無2個verbs可以痴埋一齊用,之後後面個verb加ing既grammar?
如果有,又係咩grammar呢?同埋有邊啲字可以咁用?

當然有, 就是動名詞 gerund, 最常見的例子: I like swimming.
常用的動詞還有: love, enjoy, dislike, avoid, finish, keep, stop, mind 等等