In the past I've done a few good trades to move Delta SkyMiles to AAdvantage miles. But it's a lot of work: one needs to continuously post trades in the hope that one or two of them are accepted. (Posting a trade on the site is a fair bit of work in filling out the forms.)

Usually, all I see are ludicrous ratios, which I've assumed are brokers or something (often the handles are nonsensical).