Which format to download ESTs for annotation with MAKER?

As you said it needs to be in Fasta format.

Here a complete programmatic solution to get all EST related to your species using conda and GAAS toolkit.

conda create -n gaas  
conda activate gaas  
conda install perl-bioperl  
conda install perl-bio-eutilities  
git clone https://github.com/NBISweden/GAAS  
GAAS/annotation/Tools/NCBI/ncbi_get_reference_data.pl --db nucest --organism species_name