Shimano FH-6800 rebuild or replace?
There are no thru axle FH-6800 or WH-6800 versions. The next/new Ultegra will have some, but as of now I don't think think any thru axle hubs or wheels that say Ultegra on them are really out in the wild at all.

This bikes uses the SCS dropout/hub/axle system. Long story short, in the past couple years it was unclear what road thru axle standard was going to become dominant and manufacturers had to make decisions with incomplete information to ship bikes on time. 12mm road thru axles with spacing of 142mm in back and 100mm in front have more or less won out, for now. SCS was a competing standard whose history you can find read more about here and here (this page also offers the rundown of wheel compatibility options, which is complicated but basically you can either use one of the few SCS wheelsets/hubs out there or use adapter dropouts, which can have issues with cassette clearance, and which may have already been done on this bike if it's got some kind of Shimano wheelset).

If it is a Shimano wheelset or hub, the situation is the same as with any other Shimano hub. They can be overhauled, ideally on a preventative schedule, and if done faithfully they can last a long, long time. If it's not done, or done too late, they can pick up damage to the non-replaceable or expensive to replace parts, making perfect adjustment impossible and starting the internal condition of the hubs on a downward spiral because of it. When you overhaul Shimano hubs, you have the option of whether to replace the individual ball bearings with new ones. Many of us do it by default but some find that just re-using them, especially on the high-end ones and only in a situation where no damage to anything is apparent, is fine. To replace the ones on the DA, Ultegra, etc hubs that use a plastic retainer, usually the convenient thing is to gently pop new ones into the retainer and re-use it, after cleaning it thoroughly.

A random seller recommending to replace the bearings on a Shimano hub could also be a veiled way of saying that the bearings feel gritty/notchy, in which case the hub internals are likely already damaged.

If they're trying to say the bike is having actual freehub issues (the freehub body itself is acting funny, i.e. getting stuck or not freewheeling at all), the options are usually either replace it, which is expensive for the higher end Shimano hubs, give it some flush/lube type attention, or go to the extreme of actually dismantling, inspecting, and overhauling the freehub body, which is an arcane thing that almost nobody does and requires some special tools.