Free-ads can be okay, but it depends on whether they are genuinely unwanted pet mice or feeder breeders or unwanted litters from unsexed mice. You really need to contact the advertiser to get more information. If you deal with a proper mouse breeder, you know you are getting better mice and the right gender!
Your single male was probably fighting with another male and the shop let it go on too long. Bad idea. Now that he is on his own, he`ll need to stay that way. You can have males neutered so that they can live with females, but not all vets do this proceedure, so it`s best to find one who does and one that is confident. It`s a risk also since mice are small, fragile wee creatures and some don`t come through the surgery.
Single males do require more human contact and enjoy it. They need stimulation and enrichment in their cage. get him a regular silent spinner wheel as mice love these!