West Ham have seen a number of bids rejected as they look to sign someone to support Antonio, who has only just returned from his latest hamstring injury.
Former Hammer Marko Arnautovic was top of Moyes's list of targets, though his wages of more than £300,000-a-week put a stop to a loan deal from China, while the club have also been priced out of a move for Sevilla's Youssef En-Nesyri.
With the Haller sale coming somewhat out of the blue and selling clubs aware of the fee involved, Moyes feels the club are left in a more challenging position.
"I think the minute people know you have a bit of money it could make it harder to go and get a deal," said Moyes.
"I think people know there's a bit of Haller money out there, and if I had it I could probably spend it, but there's not a lot of quality or quantity out there at the moment.
"In management the ideal situation would be to buy before you can sell if you have something lined up. But we didn't see Seb going in this window. It came a bit out of the blue to us but it's something we had to make a quick decision on, and we thought it was the right thing at the time.
"Because we're in a pandemic you're not seeing big transfers. Morally this time it's probably the right thing. We're talking about so many football clubs going out of business, there's still no crowds back at the games, so I think we all have to be a bit mindful of that."