I'm gonna to ignore all of the above for a bit. No one is being civil.
Goods on a market (including houses, chewing gum, and busses, converted or otherwise) have no intrinsic value. Instead, every individual in that market has a value range for that object. There are two types of participants in a market: sellers and buyers. Sellers drive supply, buyers drive demand.
Unique items like most converted buses are hard to set prices for. They fall into a few categories of good (recreational vehicle, bus, hobby project, etc), but they're not fungible. (You can't replace one with another without getting a distinctly different good). Features of that unique (non fungible) good add or subtract to their value for each member on the market. They might affect the price in opposite directions for different individuals (bunk houses for parents vs retiree). There are also hidden features that are part of human psychology: a seller might value it more because they like it, or because they put hours into it and want to be compensated for it, or they might want to get rid of it because it reminds them of their ex, or they might need the cash so selling quickly has value to them.
So when a seller puts a non fungible good on a market they're going to price it in an attempt to meet their goals: getting the most for it in a timeline that works for them. Your only reasonable tactic as a buyer is to try and guess where their floor is and offer somewhere just below it. The seller has already done this, but placing it just higher than the max they think they can ask for while meeting their goals.
There's one extra complexity. Sellers value range is affected by the buyer. An unpleasant, stingy, or particularly attractive buyer can contract and expand the seller's value range. So be nice.
All this to say: the price a person has listed their house, car, or bus conversion for is not actually fixed for a reason. Try offering what you think it's worth. Keep in mind that the seller might feel offended (even though economists agree they shouldn't be) if your offer is too low. And if you're selling don't be offended at any offer.