Aw, c'mon, Rent Share.
I think it is noteworthy to point out that two of your identified grammatical errors are not actually errors at all.
Hi - I am not new to timeshare but am new to TUG. . This sentence is perfectly correct except for an "s" missing at the end of "timeshare." The "I" you inserted is unnecessary and no more correct
than leaving out the "I."
In the past week I have messaged two people with postings expressing my interest and neither have responded. That sentence is correct as it stands. Yes, in modern times people may be "messaged." They may also be texted and emailed. As in "I messaged you, then texted you, then emailed you and no reply. May I inquire as to why?"
I also think technically it should be "a while" in that particular usage. However, the first post of the OP in no way sounded like someone for whom English is not a native tongue.
You mis-spelled "grammatical," BTW.
H