As far as I know, it will not ruin PR of your website. However you should check the traffic on the webpage and accordingly decide the price. Most of such Advertising sites are not reliable and it is not easy to trust on them. So be alert, best luck!
It is better that you go through the website details before you sell a link to the website. It is not necessary that the website won't be legal or have some problems as it is asking for a link. It can affect your SEO rankings as outgoing links have an affect on the rankings.
I received an offer from an advertising site who wants to pay me to put their client's links on my high PR site. Is this going to ruin my PR?
I don't think it's going to ruin anything. Any paid advertising is just that, Paid advertising. If you feel compelled then ask to review the ads prior to accepting them if you feel the content is inappropriate.