The value in my opinion is set if the buyer can honestly gauge the truth in the sellers words. The seller's attitude goes a long way & it means a lot when the buyer can be backed by the clerk when something needs to be addressed after the sale was made (warranty claims, services, other applicable terms/conditions) .

More times than I care to remember, whether from my own experiences or seeing other's, a bicycle is bought & the retailer says a lot up front & shines the best they can with assumed promises (tune ups, adjustments) but then later when the customer has a need for a tune up, the store always tags in an associated invoice for things "not covered" . It's one thing to make the customer aware before doing the work or tossing on parts, but that rarely if ever happens. The shop assumes the customer will pay up no matter what & just racks up the invoice with things surprising the customer at the time of picking it up.

No one should work for free or eat the cost of parts, but it goes both ways. The customer should not be assumed they will pay for something that was not agreed to upfront. Too much fine print.

Private sales is a totally different situation. If you're relying on selling used parts/bicycles & are not a legitimate business, then maybe look into opening up a business or jumping ship of the side-work before it sinks your livelihood?