I find the fact that in this day people can own two letter domains absolutely staggering, based on rarity, those should be worth millions I guess?
Here’s a reseller with a variety: https://1-single-letter-domains.com/
These guys run a bunch of services on x.xx domains: https://o.ee/services/ like c.im, r.nf, p.lu.
The cost was about 600 USD and was fun, but problematic as it failed to be accepted as valid email address on many websites.
There are still quite a few XX.XX left, but mostly just under obscure cTLDs (unless you are willing to consider IDN/Unicode domains under .ws or similar)
One simple thought - it’s just an email answer, not a contract/obligation that you have to sell it at particular price, you can change your mind at any time.