The 21-year-old has been a target for Reds boss Brendan Rodgers for some time and while there have been talks between the two clubs they have yet to agree a fee.
Liverpool's improved B£25million bid for Adam Lallana has already been rebuffed by Southampton, who will not consider selling anyone until they have a new manager in place, and now Castro is talking up the price of Moreno.
"It's true that there's interest from Liverpool and we're waiting to see if that turns into a reality, which right now, it isn't," the president told the club's television channel.
"What I'm clear about is that we'll not force anyone to stay at Sevilla.
"Whoever wants to leave can do so - but we'll be the ones setting the prices.
"This club is experiencing a time of economic stability and we don't need to sell.
"We're going to be firm about this and we'll only consider selling outside the market, which gives us the opportunity to make our team competitive."
With a World Cup just around the corner, Rodgers is trying to get his business done early but aside from Rickie Lambert, who Southampton let go for a fee of B£4million, deals for Lallana, Moreno and Bayer Leverkusen's Emre Can are far from completed.
Lambert's arrival on Monday has bolstered Liverpool's attacking options but fellow forward Fabio Borini, who spent last season on loan at Sunderland, does not intend to be forced out by the England international.
"Does Lambert's arrival close the door on Fabio Borini? No, I don't think so," Borini's agent Marco De Marchi told tuttomercatoweb.com.
"We have received no communications in that regard.
"In this last season, Fabio gave a considerable contribution towards keeping Sunderland up.
"He was praised by everyone, especially by the Liverpool manager Rodgers.
"So, after his holidays he will go back to Liverpool to prepare for the new season wearing the shirt of this historic club.
"He wants to compete with everyone and be an extra resource for the coach and the team.
"Borini could be of use to any team, but no-one has contacted us and his desire is to show what he's got at Liverpool."
- Sports & Recreation
- Brendan Rodgers
- Adam Lallana
- Rickie Lambert
- Fabio Borini