Converting Pound to Market?

[Abronsyth]

OK, so far I have managed to get it to determine cost based on rarity, and subtract that amount when the user buys the pet. I can also make it add that rarity-based-amount as soon as the seller chooses to send the pet away. However, I am having issues with getting it to add that amount to the seller's account once someone else buys the pet. Here's my code so far;
pound.php adopt function

PHP Code:

            if($this->settings->cost->active == "yes"){
                $adopt = new OwnedAdoptable($pound->aid);
                        if($adopt->getRarity() == "Common"){$sellvalue = 50;}
                        elseif($adopt->getRarity() == "Uncommon"){$sellvalue = 75;}
                        elseif($adopt->getRarity() == "Rare"){$sellvalue = 100;}                
                $cost = $this->getCost($adopt);
                $mysidia->user->changecash(-$sellvalue);
                $seller = $mysidia->db->select("pounds", array("lastowner"), "aid ='{$adopt->aid}'")->fetchObject();
                $sellermoney = $mysidia->db->select("users", array("money"), "username='{$seller}'")->fetchObject();
                $profit = $sellermoney + $sellvalue;
                $mysidia->db->update("users", array("money" => $profit), "username='{$seller}'");
                $this->setField("cost", new Integer($cost));
            } 


If anyone notices some major flaws I'd really appreciate it if you could share :) I'm running into this error:
Catchable fatal error: Object of class stdClass could not be converted to string in /home/inekelmy/public_html/pound.php on line 85
This is line 85:

PHP Code:

$sellermoney = $mysidia->db->select("users", array("money"), "username='{$seller}'")->fetchObject(); 


It's subtracting money from the buyer, and adding the pet to their account, but the seller is not getting money from it.