How do I record a sale of an asset?

create an income account called gain/loss on asset sales

then it depends, if the asset is subject to depreciation, you calculate and post partial year depreciation
then journal entries (*** means use the total amount in this account)

debit asset accumulated depreciation***, credit gain/loss
debit gain/loss, credit asset account***

deposit the check received for the sale, and use the gain/loss account as the source (from) account for the deposit

If the asset is subject to depreciation for fed taxes, and you did not claim depreciation expense, you need a tax accountant, the IRS says that whether you claimed depreciation expense or not, you have to figure gain/loss as if you did claim it.