How do you go about selling sports memorabilia?

Well, let's see...

If there is a collectable card store that specializes in sports cards, you could check with them if they'd buy it. (They might request authentication of the signature.)

Could check with a local auction house to sell it on consignment...

Then there's always eBay...

A lot kind of depends on what it is, like who the painting is by, what the painting is, and who it's signed by and when the person signed it. There's ebay. There are also specialty places that deal in sports memorabilia specifically autographed stuff, like Steiner, (mostly they source their own but they may take other things, depending).

There are also specialized autograph places (we have one here, high-end mostly, deals in all manner of collectible autographs), and your big auction houses, which could be interested, depending. Some will offer free appraisals, some offer appraisals for a fee. If you can share more details, that might help. A LeRoy Neiman original painting of a specific championship event signed and dated by the winner is a different thing than a painting by a total unknown that happened to be signed by someone at some point, etc.

He did the painting?

Start out with the fact that sports memorabilia, has to be pretty special to have much value nowadays. Best bet is consignment at a sports card store, who will tell you if it is a treasure or a dust collector. If they make an offer to buy it outright, it is likely they will start out offering 50% or less of what they think they can sell it for.

Not to be crass, but now would be the time to investigate that, yeah.

I'd contact Steiner Sports and ask for guidance, and Christie's sports department. There's a NY-based person midway down the page with contact info. If it's not something either would deal in, they can probably send your friends in the right direction.

Ebay may not be the worst idea, but I'd get at least a few opinions before doing anything.

ETA - I know he signed a ton of stuff, but I think his paintings are more rare and thus I think it'd likely be worth more, plus the timing. Could be the market is glutted though, who knows. Hence, experts.

One of the things I used to do, two decades ago, was go to auctions and bid on various things for some brilliantly talented antiques dealers I used to know. If they turned up at an auction and started to bid on anything, other dealers in the room would notice and bid too, knowing it was something worth bidding on. So they sent me along instead, so that the price didn't get driven up.

The things I bought for them, in country auctions across the south of England, would then be cleaned up a bit and sent to London houses where they'd sell for many times more the amounts I'd won them for. It was astonishing, every time, seeing those things sell.

I learned from working with them that specialist auction houses are great at valuing things, and can mostly be trusted; what you need is to find a place to sell your item which is populated by people who want things like the thing you're selling; and that if your item has provenance, you will almost certainly do better with it.

If you have any documentation at all which proves that Ali painted this plate, dig it out. It doesn't have to be legally recognised: a letter from him to your friends would be fab (that would add a huge amount of value, as it would be significant in itself), or pictures of them together. Anything like that that can be found will be useful.

What a very lovely thing to have.

It is disturbing that you're referring to 2001 and past that as "all these years ago," heh.

Hang on. It was before my eldest was born, and he's going to be 21 this year. So perhaps it was 25 years ago. Which is much more than half my life ago. Good grief.