Hi Gemlaco! -- The only thing I can offer here is this:
When carefully comparing the html code in question (from the offending listing), there appears to be a difference between it and the equivalent code section of another of your listings (which correctly displays the official PayPal Seal).
The difference is that in the 'bad' listing's code... I see that there are several of these: [ &_gt ] and [ &_lt ] (as in "greater-than" and "less-than" ) inserted in the html code...as opposed to (and in place of) the actual " > " and " < " delimiter characters which I found properly showing-up in the html of your other good listing.
Now I'm sorry to tell you that unfortunately...I'm not an HTML guy! So I have no clue as to how, when, or why the difference occurred. My wild-@ss-guess is that somewhere along the line...the ">" and "<" characters got mis-parsed (or something like that). But obviously this doesn't solve your problem...or suggest who to look to... for a 'fix'.
Anyway... I offer my observation only in the hopes that someone out there who is far more qualified than I, might see this as a clue (however small it is). Perhaps you could/should file an official support-request ticket from the helpful tech-staff at Auctiva. You might mention the difference in code sections as well. --- I sincerely hope my two-cents-worth doesn't just muddle the issue for you!
cheers!