Atticofunclefred
The hold may be for the following reason:
You have been an eBay member for less than 6 months, and you sell an item for more than $100
I don't know what you sold your science kit for, but if it was over $100 then that is the reason they are holding it.
I often have pending payments but that is usually because the buyer has transferred funds from their bank account and it needs 4-5 days to complete the transaction. It is definitely not because of the item I am selling. My seller account has been around for 8 years.
In order to get positive feedback, the buyer needs to receive the item. In order for the buyer to receive the item, it needs to be shipped. In order to ship it you need the funds. Catch-22 situation. However, in this case, the funds are there and the buyer has paid, so if the item is received as described and feedback, left you will get your money as it is not the buyer holding it up.
In other words, in the eyes of Ebay and Paypal, if your item is on the up and up, you will get your money.
I am not sure what I would do, really. Good luck!