Ballpark price I should be paying for LifePO4 from places such as Ebay, also savings questions ?

Hi, Im looking to build an offgrid system on a boat, but honestly, I have no idea what the market price is for this stuff outside the drop in solutions, like battlerborn ren, ect. Im new to batteries but I am tech savy and mechanically inclined so Im picking it up pretty quickly. What is the a solid price currently for Lifepo4s on the used market ? What ballpark price is too much, and ball park figure, would you be wary off if someone is selling off a battery ? Can I actually get siginfiant savings compared to a drop in replacement that is fresh off the shelves? Looking to build a 24v