Going back 25+ years in my experiences with all things sold and traded secondhand, "OBO" has been used to abbreviate either/or/both of the following:

1) "Or Best Offer"
2) "Or Better Offer"

Note that to students of English, those do NOT mean the same thing!

"Asking 5K, but possibly willing to settle for less if that turns out to be the best offer I can get" = usage 1.
"Asking 5K, no less, but will sell to anyone who offers better [which may or may not even mean cash per se]" = usage 2.

Frustratingly for buyers, both are correct and fair extrapolations of the now ambiguous acronym "OBO". You have no way to know if a given seller is only/more familiar with one meaning by default, or merely playing/choosing to suit their current intent.

FWIW, I consider Usage 2 to be the douche move - because yes, just list a flat price, or an auction, or whatever - but I do understand it because I have been "outbid" by others who apparently just had to have something more than I did. Flipside, which offer might I take under certain circumstances? That's Capitalism, baby!

So the definition of OBO is not worth arguing over. While real world use is annoyingly inconsistent, both are correct and practical. As a buyer all you can do is be aware of the market at large and offer your best price. If the seller doesn't accept, make a counteroffer and be better prepared next time.