im selling my motorcycle what should I tell this guy?

buyer a: $3500 offered at 1:00pm
buyer b: $3500 offered at 6:00pm
buyer a : counter offer $3800 at 6:30pm
I want to sell to buyer (A) he called first and Its his first bike I know how he feels

What should I tell buyer b just in case buyer (A) defaults

buyer (B) wants to come tomorrow @1:00pm
buyer(A) cant make it till 6:30 pm

I told buyer (A) that I would hold the bike for him but I don't want to lose buyer (B) in case buyer (A) defaults

if buyer (A) defaults I dont want to tell buyer (B) because he will offer lower if there is no competion

the bike has only been for sale for 3 days