Time during which steam prevents the sale of items unless I buy more games

Solution 1:

Your account must have purchased a game between thirty days and one year ago. This means that if you don't purchase anything for a whole year, you will lose access to the marketplace.

From the Steam FAQ:

Purchase Limitations Your account must have a successful purchase older than thirty days, but not older than a year in order to access the Marketplace. The purchase of wallet funds, games, DLCs and software meet this requirement. Retail CD key activations and gift purchases do not satisfy this requirement.

And the Spanish version:

Limitaciones en las compras Para acceder al Mercado de la Comunidad tu cuenta debe tener una compra realizada hace más de treinta días, pero no mayor a un año. La compra de fondos de la Cartera, juegos, DLCs y software te permite cumplir este requisito, pero no la activación de claves de producto o la compra de regalos.

https://support.steampowered.com/kb_article.php?ref=1047-EDFM-2932&l=english https://support.steampowered.com/kb_article.php?ref=1047-EDFM-2932&l=spanish

Solution 2:

If you're asking how long you have to sell your items once your earn them, the answer is forever - you don't lose the right to sell things unless you become trade-banned for doing something like scamming.