Question: What is the effect of a sale of asset on stock price?

In business, economics or investment, market liquidity is an asset's ability to be sold without causing a significant movement in the price and with minimum loss of value.
Money, or cash, is the most liquid asset,
and can be used immediately to perform economic actions like buying, selling, or paying debt, meeting immediate wants and needs.
However, currencies, even major currencies, can suffer loss of market liquidity in large liquidation events.
For instance, scenarios considering a major dump of US dollar bonds by China or Saudi Arabia or Japan, each of which holds trillions in such bonds, would certainly affect the market liquidity of the US dollar and US dollar denominated assets.
There is no asset whatsoever that can be sold with no effect on the market.
An act of exchange of a less liquid asset with a more liquid asset is called liquidation.
Liquidity also refers both to a business's ability to meet its payment obligations, in terms of possessing sufficient liquid assets, and to such assets themselves.

Answer:

It really depends on the asset and circumstances. All else equal from an accounting standpoint its a zero sum gain, sell an asset and gain cash or something in exchange.

An Asset sale reduces the value of a company and thus its stock price/valuation.

It depends on the asset, its value and its sale price. An asset usually has a benefit to a business (like property) so the cash may not be as beneficial. On the other hand the asset may be being sold at a really good profit and the interest on the cash, or the cash itself may be better employed by investing in another, better asset. Or paying off expensive debt.