You have Best offer on your listings but dont have to accept offers that are not enough.
You can put in an amount on when you list to say what is the lowest you are willing to accept then ebay would automatically refuse lower offers.
When you have best offer on a listing you will often get some that try it on but you can just reject the offer or just ignore it
You can also counter offer to a price you are willing to accept
You could also get offers higher than the start price
