Value of Encyclopedia Americana?

Encyclopedias are in a class by themselves when it comes to value as having an older set does not always mean it has more value than a later set.

This may be true with your set but buyers are sometime fickle about certain things and have their own reasons for even looking at a set of older reference books. Everything is available online (usually free - this set is available to research online) so a potential buyer has to have their own reasons for buying older sets.

Since no one can tell what reasons someone is interested in seeing a seller has to try to appeal to potential buys in a number of ways. This usually includes several very good pictures - pictures of full set, individual books (showing any and all flaws), inside pages of front page showing all publication information, pictures of interesting things (especially maps).
If posting on eBay you can post up to 12 pictures for free so it is best to use all 12.
Provide a very good description and allow shipping as well as free local pick-up.

Shipping is tricky and sets may require 2 or 3 boxes so all of this has be considered before listing a set. Encyclopedias should be shipped USPS Media mail.

You can always check listings on eBay to see if a similar set is listed but just remember - asking prices are just that - a price the seller would really like to receive for their set. Research is difficult to narrow down as most sellers do not list vital information so you have to just skip over the unrelated items in your research.
You can place a watch on any set if you wish to see if it sells.
Books are notoriously slow sellers.

www.ebay.com/.../i.html?_from=R40&_nkw=Encyclopedias...

Sold items are the only reference that is dependable for current value.
Books cannot usually be compared as there can be many factors that determine value but it can be used as a 'reference' for value.

Check sold listings often.

I did see a 1924 set in the sold listings but have no idea whether it compares with your set. This shows it sold at $89 but it was sold as best offer so there is no way to tell the actual price a buyer paid (but less than $89) and we have no way to tell how long this set was posted on eBay - weeks/months/?.
www.ebay.com/.../303260608345?hash=item469bbda759...

here is a link to eBay sold listings.

www.ebay.com/.../i.html?_from=R40&_nkw=Encyclopedias...

I would suggest you take good pictures and post your set on local sites like Craigslist, LetGo, OfferUp, Facebook groups/marketplace and list it at a higher price - maybe $300 and lower it if no one shows an interest.
Post it on more than one site and list it with shipping cost or local pick up.
Listing on eBay can be very tricky for new sellers but you can do this also.