Historically registering a credit card is the most likely thing you can do to get your Buyer Activity Limits significantly increased in the short run. See https://www.ebay.com/help/buying/buying-limits-restrictions/buying-limits-restrictions?id=4012&st=12... for more information and instructions.
You also might be running into your Open Transaction Limits (they used to be described in the above linked page, but got lost in a reformat). To get below your limits you need to pay for the outstanding unpaid items that now count towards your limits, but that might not be immediately possible because eBay counts items that you committed to buy but cannot pay for at this time against your limits:
1. Active auctions that you are the lead bidder on at present (including the one you are trying to bid on and it can take a little while after you are outbid before the server checking on whether you can bid knows that you are outbid); to minimize the effect of these wait until near the end of the auction to bid only one time at your TRUE maximum you are willing to pay for the item, and then pay immediately after it ends, to minimize the time it counts towards your limit. You can't pay until you win, so don't use up your open transactions on bids you can't pay for until hours or days later.
2. Transactions cancelled at your request (or due to problems with your address). Note that these also can result in lowering the limit, so make sure you are going to buy (and CAN buy, that the item ships to your location) BEFORE you commit to buying.
3. Unpaid Item Strikes. Again, don't bid/commit to buy unless you KNOW you can and will pay for every item.
To get these and other buying limits raised or removed in the long run you need to establish a pattern of reliably paying for what you commit to buy.
