The superproducer surprisingly posted a listing on eBay Wednesday evening ... offering his 5% publishing stake on the track "Delresto (Echoes)."  It's an unprecedented move, but one that seems to be working -- the song has a current bid of $15,000!!!

Travis hands-down has the biggest rap album of the year ... topping Billboard's 200 album chart 4 times now, and the project was certified platinum after just a month.

A song like "Delresto" may prove to be even more valuable down the line on such a notable album, but HB tells TMZ Hip Hop he literally doesn't GAF about hoarding stocks and wants the money A-S-A-P!!!

To sweeten the deal, HB's also including the AKAI keyboard he used to make the beat in the sale, a real steal for music-makers.

It's not like HB is in the minority with his thinking ... rappers have been selling their catalogs more frequently these days, and some are even blatantly lying about offers in hopes of generating interest.

Nelly's one of the real deals ... we broke the story back in the summer when the "Country Grammar" rapper became $50 million richer for parting ways with just half his catalog.

So, HB's just striking while the iron is hot.

He and Travis have made plenty of music and money together, their "Sicko Mode" classic literally went Diamond this year!!!