The title says it all, really: where can I buy a good second hand bike (not a motorbike) in Hua Hin, preferably one with at least a few gears. 

Cheap enough new on Lazada, Shopee or Tesco.

Indeed, there are bicycles in the basement of Market-Village
Maybe ask them if they don't have used bikes.
There was a bicycle dealer, towards N ° 43, going towards Cha-Am.
There were used bicycles.
If the store still exists, you have to see it, the bikes are on display, too, outside.

Unless you're wanting to purchase something exceptional, New bicycles are not at all expensive and available at most (if not all) popular retailers.

 

If you're looking to 'bulk buy' with a view to opening future opportunities. Just look for places that rented bicycles, a year or two ago. For sure, you'll find one that needs to expand their cashflow, at this time.