I'm a real estate Broker in California, as well as an STR manager/owner. I keep my Brokerage website separate from my vacation rental site. There are two main reasons for this, both around SEO. If you have STRs and sales it will "confuse" the search engines as to what you are and what your site's focus is. This will get even more confusing, especially for the consumer, if you have long-term rentals on there as well.
The second reason is, your site's "avatar" (aka who you're trying to get as a visitor). The person who is shopping for a traditional rental or to purchase a home is much different than the person looking for a two-night stay somewhere. When designing your site, they need to attract the right customer and your content needs to reflect that. Even if you had a subdomain like "purchase.mysite.com" it could still be dicey. The guidance I've read is to have these be unique sites and leave subdomains for specific silos of information, like "calif.aaa.com"
Evolve does have a sales page buried on their site, but when you click on the listing for the property it takes you to the rental page. In my view, that is not a good way to do it as it is confusing.
How I would do it is list the rental on your sales site with a link to the rental page/site in case people are interested in buying it as a rental. I'd also link from the STR site to the sales site with a generic "some of our rentals are up for sale, click here to see a listing" type of link. You don't want to list directly from the STR property page as that could hurt rentals (who wants to rent a place that may be sold out from under them).
