Value of 1911 United Editors Perpetual Encyclopedia?

I cannot find out very much about this set except that it is available as an E book free to download.
There were a couple of volumes listed individually on Amazon and seller was asking $25 each so that may be a way to go but may be a long slow process.

www.amazon.com/.../B000EOR5VO (Affiliate Link)

I can only suggest you try listing it for sale at whatever price you feel it's worth - maybe even a high price if the books are in good condition - start at $250 plus shipping or will consider best offer and also available for local pick up.

Shipping can be by Media mail so it should not be too expensive (probably 2 or 3 boxes).
eBay is a good site but can be tricky for a new seller so you might want to think about Craigslist, Facebook Marketplace, letgo, offerup or any other site you're familiar with.
You will need several good pictures of outside covers and also of several interesting inside pages and also publication page.
Post on several sites and if no one makes an offer you can lower the price - just remember - books of any kind are generally slow sellers.

All books can be worth something but it all depends on what people are willing to pay for them. A lot of people can now download free PDF files of these older books and have them stored on their computer to make it easier to read and look things up in them. A lot of people right now are trying hard to downsize their homes and get rid of a lot of things like this. You can try first off to speak to an antique bookseller in your area and see if they are willing to take the books and sell them for you. An auction house is yet another good place to sell off books like this.

Listing them on Craigslist in your area is a good way to go. I would be careful listing them online to sell because the price of shipping off a complete set of books like this is very expensive. You can also try Facebook Marketplace and put in the ad that it is pick up only. The market is rather open at this time because there is not a lot of people trying to sell these books. You can basically determine what you'd like for the books and list them that way. You can always adjust your price if a person wants them.