Can you tell if the sword has a fuller (groove) running the length of the blade on either side? I also wonder if the sword had a Pommel at the end of the handle which may have rusted off and been left in the hole where you dug it? Pommels are very helpful for identifying and dating ancient or medieval swords.
Also, regarding the legal grey area, many antiquities excavated in Bulgaria and the wider Balkan region are claimed to be "from an old collection" when they are sold, which might be a loophole they use to traffic recently-excavated antiquities by suggesting that the items were excavated before the laws were passed, which is hard to prove either way. I'm not recommending that you do anything illegal, but I also don't think it's fair that the state can claim ownership of everything you might dig up. Many antiquities from your part of Europe get sent to dealers in Germany and the Netherlands who then say they are "from an old German collection" and sell them on internet auctions all across the world. If this sword was to be sold in this manner, I would expect it to get a price between 500 and 1,000 USD.
Because Viking antiquities are in high demand right now, many sellers claim that swords, spearheads, etc of uncertain Medieval origin are Viking to get more money. Likewise, ancient Roman items are in higher demand than Celtic items, so because a basic spatha (broadsword) used by the Romans versus one used by the Natives would be very similar in appearance, such dealers might call an uncertain Iron Age sword an "Ancient Roman Spatha" to get a higher price. I am very far from an expert, but collecting iron artifacts is a hobby of mine so I've seen how the market works over the past 15 years or so.
Anyway, it is a lovely discovery, and if it were my find I'd keep it forever!

Please share some of your other discoveries, ancient European metal artifacts are a favorite of mine.