It's going to depend on the domain name, amount of traffic it had, if it still ranks in the search engines, how popular the name is and a bunch of other factors. But yes, they CAN be profitable if you know what you are doing.
Everything is down to the merit of the domain and what you are targeting.
If you are targeting keywords to suit your business then you may benefit instantly. If you buy names based on attributes eg DA or backlinks then those need to be closely looked at if the name itself doesn't attract business. If you already have an advertising network you can direct traffic then buying names and milking the traffic until fail to offer a return. I would strongly suggest unless buying thousands of backlinks or reciprocal linking with relevant sites before you think discovering gold. I have also seen a lot of fudged statistics at the most popular expired domain marketplaces so would look for a second opinion eg use another tool and check the backlinks and remember will be be dropping off if not hosted.
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