I propose that either;
Any sold building can be unsold within a certain amount of time (30seconds? 1 minute? 5 minutes? 30 minutes? you decide! we'll take what we're given and give a good smile while we're at it

Or
To sell a building (not including walls ofc. That would get old fast huh) you would have to complete a simple math sum, such as, 3+5=?
I think this would relieve some of the stress felt by Lords and Dukes alike when on clan maps, especially during laggy times.