Hi Amy,

Yes there really was a High Sierra in Lake Tahoe from 1983 to 1990. The chronology was:

Del Webb built the Sahara Tahoe at South Shore in 1965

The name was changed to Del Webb's High Sierra in 1983

It was sold to William J. Yung in 1990, who changed its name to Horizon Hotel and Casino, which name it carries today.

Above info from the indispensible Art Anderson's "Casinos and their Ashtrays".

High Sierra had only one set of chips in use, according to TCR. Values of the $1, $2.50, $5, and $25 are $10-14, $20-24, $15-19, and $50-59 respectively.
You can see what the first 3 look like below.

Hope this helps answer your questions of value and authenticity. Don't know the seller you mentioned, at least not by his email moniker.

DonL