I used to do this a lot, within London. You are in London too right? People were always amazed how much I could bring, on a bus or the tube.
I got packing to fine art and concentrated on smaller items, jewellery, scarves, etc, so I could bring the maximum stock in the minimum cases, though I did bring clothes as well. I had two big wheely suitcases, with a small rail taken apart and put in one of the outside pockets, sticking up. I would pull a case in each hand, then I'd have a small back pack or cross body bag as well. If I had to get on an escalator, I got good at getting one suitcase behind me and one in front. Sometimes, I'd just take jewellery in one suitcase, which was fairly easy.
I would only do it if I could find a good route to the fair. Generally buses are easier than trains, less walking and less stairs. If you use TFL journey planner, you can specify that you need step free access, and see if there's a route you can take (you can even say that you can use escalators but not stairs). There is also a site for disabled people that gives you specific information about every train and tube station, eg exactly how many stairs there are at any point, to get from the platform to the surface, or to change trains. I'd allow plenty of time for the journey so I didn't get exhausted!
It was hard work, I've got to admit, but if you pack right and get your journey worked out, it is doable.
Of course, the other option I sometimes did was take a taxi. It adds to the cost of the fair, but if it's a good fair and nearby, it was sometimes worth it.