Definitely Francis Huntington – maybe. Huntington owned a lot of property on the north side of Asylum Avenue, and that property stretched from Huntington Street to east of Sumner Street, so in all likelihood he owned the land on which the church now sits. The Courant includes B. E. Buck in the transaction, but it's unclear what role he played -- Huntington's partner? seller of a separate tract of land? -- or even if he was actually involved at all. Hopefully, a quick trip to the land records will clarify this.
Presumably the members of the committee empowered to buy the land were involved, but it could also have been officers of the society.
At the February 6, 1864 meeting, two sites emerged as favored: a spot between Spring and Garden Streets and a spot on Farmington Avenue near Imlay Street. William Turner, who had been principal at the American School for the Deaf, also offered the property he owned on Farmington Avenue, directly across the street from the ASD and next door to West Middle School. This location was just east of the present-day Asylum Place. By February 19, all of these sites were rejected in favor of the present location of the church.
The best indication of what had been on the land comes from an 1855 map of Hartford. That map shows a house belonging to F. R. House on what would become Asylum Hill Congregational Church, roughly where the parish house would eventually be built in 1903-1904. Other than this map, the only record of F. R. House that I've found is that in 1863 he reported for duty with the Army of the Potomac, and at that time, he lived on Sigourney Street, not Asylum Avenue. Sumner Street was opened in 1859, and it established the eastern boundary of the land the Asylum Hill Congregational Society purchased. Huntington Street, however, was not opened until 1864, just after the society bought the land.